Learn Multi Platform 6502 Assembly Programming for Monsters

Welcome To the Dark Side!... I grew up with the Amstrad CPC, and I started learning Assembly with the Z80, however as my experience with Z80 assembly grew, I wanted to start learning about other architctures, and see how they compared!
The 6502, and it's varients powered many of the biggest systems from the 80's and 90'... From the ubiquitous C64... to the Nintendo Entertainment System, as well as the BBC Micro, PC-Engine and Atari Lynx... even the Super Nintendo used a 16 bit varient of the 6502 known as the 65816
The 6502's origins are somewhat odd, a cost reduced version of the 8-bit '6800'  (which was the predecessor to the venerable16-bit 68000)... the 6502 sacrificed some functions for a cheaper unit price, which allowed it such wide support... the 6510 which powered the C64 had a few added features...
A later version, the 65C02 added more commands (Used in systems like the Apple IIc and the Atari Lynx) ... and HudsonSoft made a custom version of the 65C02 with even more features, called the HuC6280 and exclusively used in the PC Engine
All these CPU variants are 8 bit, and the basic 6502 command set works in the same way on all these sysems, and it's that instruction set we'll be learning in these tutorials...
These tutorials will be written from the perspective of a Z80 programmer learning 6502, but they will not assume any prior knowledge of Z80, so if you're starting out in assembly, these tutorials will also be fine for you!
In these tutorials we'll start from the absolute basics... and teach you to become a multiplatform 6502 monster!... Let's begin! the 6502

The 65C02 die If you want to learn 6502 get theCheatsheet! it has all the 6502 commands, it also covers the extra commands used by the 65c02 and PC-Engine HuC6280

Table of Contents

The 6502

Beginners Series - lets learn the basic 6502 commands by example!

Lesson 1 - Getting started with 6502

Apple IIe

Atari 800 and 5200

Atari Lynx

BBC B

Commodore 64

Super Nintendo (SNES)

Nintendo Entertainment System / Famicom

PC Engine

Vic 20

6502 Getting started

6502 Tricks

What is the 6502 and what are 8 'bits' You can skip this if you know about binary and Hex (This is a copy of the same section in the Z80 tutorial)

The 6502 is an 8-Bit processor with a 16 bit Address bus!

What's 8 bit... well, one 'Bit' can be 1 or 0

four bits make a Nibble (0-15)

two nibbles (8 bits) make a byte (0-255)

two bytes (16 bits) make a word (0-65535)

And what is 65535? well that's 64 kilobytes ... in computers Kilo is 1024, because four bytes is 1024 bytes

64 kilobytes is the amount of memory a basic 8-bit system can access

6502 is 8 bit so it's best at numbers less than 256... it can do numbers up to 65535 too more slowly... and really big numbers will be much harder to do! - we can design our game round small numbers so these limits aren't a problem.

You probably think 64 kilobytes doesn't sound much when a small game now takes 8 gigabytes, but that's 'cos modern games are sloppy, inefficient,  fat and lazy - like the basement dwelling losers who wrote them!!!

6502 code is small, fast, and super efficient - with ASM you can do things in 1k that will amaze you!

Numbers in Assembly can be represented in different ways.

A 'Nibble' (half a byte) can be represented as Binary (0000-1111) , Decimal (0-15) or  Hexadecimal (0-F)... unfortunately, you'll need to learn all three for programming!

Also a letter can be a number... Capital 'A'  is stored in the computer as number 65!

Think of Hexadecimal as being the number system invented by someone wit h 15 fingers, ABCDEF are just numbers above 9!

Decimal is just the same, it only has 1 and 0.

In this guide, Binary will shown with a % symbol... eg %11001100 ... hexadecimal will be shown with & eg.. &FF.

Assemblers will use a symbol to denote a hexadecimal number, some use $FF or #FF or even 0x, but this guide uses & - as this is how hexadecimal is represented in CPC basic

All the code in this tutorial is designed for compiling with WinApe's assembler - if you're using something else you may need to change a few things!

But remember, whatever compiler you use, while the text based source code may need to be slightly different, the compiled "BYTES' will be the same!

Decimal 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ... 255 Binary 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 11111111 Hexadecimal 0 1 2 3 4 5 6 7 8 9 A B C D E F FF

Another way to think of binary is think what each digit is 'Worth' ... each digit in a number has it's own value... lets take a look at %11001100 in detail and add up it's total

Bit position 7 6 5 4 3 2 1 0 Digit Value ( D ) 128 64 32 16 8 4 2 1 Our number ( N ) 1 1 0 0 1 1 0 0 D x N 128 64 0 0 8 4 0 0 128+64+8+4= 204            So %11001100 = 204 !

If a binary number is small, it may be shown as %11 ... this is the same as %00000011

Also notice in the chart above, each bit has a number, the bit on the far right is no 0, and the far left is 7... don't worry about it now, but you will need it one day!

If you ever get confused, look at Windows Calculator, Switch to 'Programmer Mode' and  it has binary and Hexadecimal view, so you can change numbers from one form to another!
If you're an Excel fan, Look up the functions  DEC 2 BIN and DEC2HEX ... Excel has all the commands to you need to convert one thing to the other!

But wait! I said a Byte could go from 0-255 before, well what happens if you add 1 to 255? Well it overflows, and goes back to 0!...  The same happens if we add 2 to 254... if we add 2 to 255, we will end up with 1

this is actually usefull, as if we want to subtract a number, we can use this to work out what number to add to get the effect we want

Negative number -1 -2 -3 -5 -10 -20 -50 -254 -255 Equivalent Byte value 255 254 253 251 246 236 206 2 1 Equivalent Hex Byte Value FF FE FD FB F6 EC CE 2 1

All these number types can be confusing, but don't worry! Your Assembler will do the work for you!

You can type %11111111 ,  &FF , 255  or  -1  ... but the assembler knows these are all the same thing! Type whatever you prefer in your ode and the assembler will work out what that means and put the right data in the compiled code!

The 6502 Registers

Compared to the Z80, the 6502 has a more limited register set...  

The Z80 has Accumulator, 3 pairs of 8 bit regsiters (BC,DE,HL), usable for 16 bit maths and 2 16-bit  indirect registers (IX,IY), it also has a 16 bit Stack pointer, and there are 'Shadow Regsiters' for special purposes

The 6502 is very different, it has an 8 bit Accumulator, two 8 bit  indirect registers (X,Y)  and an 8 bit stack pointer... it also has a 16 bit Program Counter... it has no Shadow Registers

8 Bit 16 Bit Use cases Accumulator A Flags F Indirect X X Preindex register , stack pointer manipulation Indirect Y Y Postindex register Stack Pointer SP Stack Program Counter PC Flags: NV-BDIZC Name Meaning N Negative 1=Negative V Overflow 1=True - unused B BRK command D Decimal mode 1=True I IRQ disable 1=Disable Z Zero 1=Result Zero C Carry 1=Carry

At a glance this may make the 6502 seem significantly inferior to the Z80, however the 6502 has some tricks up it's sleeve!... Where as the fastest command on the Z80 takes 4 ticks, on the 6502 it takes only 1... and the 6502 makes up for it's lack of registers with superior addressing modes!

Special Memory addresses on the 6502

Compared to the Z80, two things are apparent about the 6502... firstly the stack pointer is only 8 bit... and secondly we have very few registers!
The way the Stack pointer works is simple... the stack is always positioned beween $0100 and $01FF...   Where xx is the SP register, the stack pointer will point to $01xx
The 'solution' to the lack of registers is special addressing options... the first 256 bytes between &0000 and &00FF are called the 'Zero Page', and the 6502 has many special functions which allow data in this memory range to be quickly used with the accumulator and other functions as if they were 'registers'!
Note: the PC-Engine has different Zeropage and Stackpointer addresses... and the 65816 can relocate them!... in this case the Zeropage (ZP) is often referred to as the Direct page (DP) From To Meaning $0000 $00FF Zero Page (zp) $0100 $01FF Stack Pointer $0200 $FFFF Normal memory (and mapped registers)

The 6502 Addressing Modes

The 6502 has 11 different addrssing modes... many have no comparable equivalent on the Z80

Mode Description Sample Command Z80 Equivalent effective result Implied / Inherant A command that needs no paprameters SEC SEC  (set carry) SCF Relative A command which uses the program counter PC with and offset nn (-128 to +127) BEQ #$nn BEQ [label] (branch if equal) JR Z,[label] Accumulator A command which uses the Accumulator as the parameter ROL ROL (ROtate bits Left) RLCA Immediate A command which takes a byte nn as a parameter ADC #$nn ADC #1 ADC 1 &nn Absolute Take a parameter from a two byte memory address $nnnn LDA $nnnn LDA $2000 LD a,(&2000) (&nnnn) Absolute Indexed Take a parameter from a two byte memory address $nnnn+X (or Y) LDA $nnnn,X LDA $2000,X (&nnnn+X) Zero Page Take a parameter from the zero page address $00nn ADC $nn ADC $32 (&00nn) Zero Page Indexed Takes a parameter from memory address $00nn+X ADC $nn,X ADC $32,X (&00nn+X) Indirect Take a parameter from pointer at address $nnnn...
if $nnnn contains $1234 the parameter would come from the address at $1234 JMP ($1000) LD HL,(&1000)
JP (HL) (&nnnn) indirect ZP The 65c02 has an extra feature, where it can read from an unindexed Zero page LDA ($80) ((&00nn)) Pre Indexed (Indirect,X) Take a paramenter from pointer at address $nnnn+X
if $nnnn contains $1234, and X contained 4  the parameter would come from the address at $1238 ADC ($nn,X) ADC ($32,X) ((&00nn+X)) Postindexed  (Indirect),Y Take pointer from address $nnnn, add Y... get the parameter from the result
if $nnnn contains $1234, and Y contained 4, the address would be read from $1234... then 4 would be added... and the parameter would be read from ther resulting address ADC ($nn),Y ADC ($32),Y ((&00nn)+Y)

LDA #val1

CMP #val2

We can test the result with the following commands

Basic command Comparison 6502 command Z80 equivalent 68000 equivalent if Val2>=Val1 then goto label >= BCS label JP NC,label BGE label if Val2<Val1 then goto label < BCC label JP C,label BLT label if Val2=Val1 then goto label = BEQ label JP Z,label BEQ label if Val2<>Val1 then goto label <> BNE label JP NZ,label BNE  label Prefix Example Z80 equivalent Meaning # #16384 16384 Decimal Number #% #%00001111 %00001111 Binary Number #$ #$4000 &4000 Hexadecimal number #' #'a 'a' ascii value 12345 (16384) decimal memory address $ $4000 (&4000) Hexadecimal memory address

If you forget the # in a command like ADC #3... you will end up adding from the zeropage address $0003 - and your program will malfunction

With VASM you do not need to put a # where it is always a number, like on jump commands or data declaractions like "DB $3" or "BRA 3"

Low and High Byte

Because the 6502 has no 16 bit registers, it's often nesassary to split an address into its High and Low byte parts, by prefixing a label with < or > it's low or high bytes will be extracted and used in the compiled code, lets take a look!

Symbol Meaning Example Result < Low Byte #<$1234 #$34 > High Byte #>$1234 #$12 7 6 5 4 3 2 1 0 anytime ASL A
BCC/BCC Dest ASL A
BPL/BMI Dest AND #32 AND #16 LSR A
BCC Dest After a BIT command BPL/BMI Dest BVS/BVC Dest Missing command Meaning 6502 alternative ADD #5 ADD a number without carry CLC

(Clear carry for add)

ADC #5 (Clear carry) SUB #5 Subtract a number without carry SEC

(Clear carry for sub)

SBC #5 (Clear carry) NEG convert positive value in Accumulator  to negative value in Accumulator EOR #255 (XOR/Flip bits)
SEC (Clear carry)
ADD #1  (add 1) SWAP A Swap two Nibbles in A

ASL (shift left - bottom bit zero)

ADC #$80 (pop top bit off)

ADC #$80 (pop top bit off)

(shift carry in)

RLCA Rotate left with wrap CLC

(Clear the carry)

ADC #$80

(pop top bit off)

ROL (shift carry in) RRCA Rotate right with wrap PHA

(Backup A)

ROR

(Rotate Ritght - get bit)

PLA

(Restore A)

ROR (Rotate Ritght - set bit) BRA r Jump to PC relative location +r
(Use instead of JMP for relocatable code) CLV Clear Overflow
BVC n Branch if overflow clear CALL NZ,subroutine Skip over subroutine command if Zero BEQ 3 Skip the JSR command
JSR subroutine Csubroutine to call if nonzero RET Z Skip over return command if Zero BNE #1 Skip the RET command
RTS  Return if zero PHX / PHY Push X (PHX does exist on 65c02)
(do opposite for PLX) TXA
PHA HALT infinite loop until next Interrupt CLV
BVC -2 LDA (zp) Load a from the address in (zp)
(not needed on 65c02... use LDA (00zp)
(do same for STA etc)

LDX #0

LDA (zp,X)

or

LDY #0

LDA (zp),Y

If you're used to the Z80, don't go looking for INC A or DEC A on the 6502 ... they don't exist either, so you'll have to CLC, ADD #1 instead!... however they DO exist on the 65C02 and HU6280 as DEA and INA

Use ASL to shift bits left, if you don't want the carry (and bottom bit can be 0)

use LSR to shift bits right without the carry

Skip over parameters 

We may call a subroutine, and pass some parameters, there are two ways we can do this

Using Zeropage Using X (takes 7 more bytes) JSR TestSub
db $11,$22,$33  ;Parameters
TestSub:
...
PLA
CLC
ADC #3+1 ;(parameter bytes+1... so 3+1)
STA retaddr
PLA
ADC #0
STA retaddr+1
JMP (retaddr) JSR TestSub
db $11,$22,$33  ;Parameters
TestSub:
...
TSX
LDA $0101,X
CLC
ADC #3  ;(parameter bytes... so 3)
STA $0101,X
BCC 3  ;Skip over inc command (3 byte cmd)
INC $0102,X
RTS INC (inc de) DEC (dec de) ADD (add bc to hl) SUB INC z_E
BNE    IncDE_Done
INC    z_D
IncDE_Done: INC z_E
BNE    IncDE_Done
INC    z_D
IncDE_Done: clc
lda z_c
adc z_l
sta z_l
lda z_b
adc z_h
sta z_h lda z_l
sbc z_c
sta z_l
lda z_h
sbc z_b
sta z_h

lda (z_hl),y

....

iny

bne fontchar_loop

inc z_hl+1

dex

bne fontchar_loop

Status Register bits

7 6 5 4 3 2 1 0 Negative Overflow Unused Break Decimal mode Interrupt state Zero Carry 1=Negative
0=Positive 1=Overflow
0=No Overflow 1=BRK occured
0=Normal 1=Dec
0=Bin 1=on
0=disabled 1=Zero
0=Nonzero 1=NoCarry
0=Carry

Get 16 bits from a Lookup Table

lookup 16 bit value A in [table] ASL A
TAX
LDA table,X
STA destval
INX  
LDA table,X
STA destval+1
16 bit value is now in destval ASL A
TAX
LSA BASE+1,X
PHA
LSA BASE,X
PHA
RTS
(because RET  adds 1 to address - you must subtract 1 from pointers in table) Lesson 1 - Getting started with 6502

I Learned Assembly on the Z80 systems, and the 6502 seemed strange and scary!... but there's really nothing to worry about, while you have to use it a little bit differently, programming 6502 is no harder than Z80!

Lets start from the basics and learn how to use 6502!

Vasm, Build scripts and Emulators

In these tutorials, we'll be using VASM for our assembly, VASM is free, open source and supports 6502,Z80 and 68000!
We will be testing on various 6502 systems, and you may need to do extra steps (such as adding a header or checksum)... if you download my DevTools, batch files are provided to create the resulting files tested on the emulators used in these tutorials.

My sources will use a symbolic definition to define the platform we're buiilding for, if you use my batch files this will occur automatically, but if you're using your own scripts, you need to define this with an EQU statement.

Here's the platform, symbol I use, and emulators we'll be looking at!

Platform Symbol Definition Required Emulator used Apple IIe BuildAP2 equ 1 AppleWin Atari 5200 BuildA52 equ 1 Jum52 Atari 800 BuildA80 equ1 Atari800win BBC Micro B BuildBBC equ1 BeebEm C64 BuildC64 equ1 Vice Atari Lynx BuildLNX equ 1 Handy Nintendo NES/Famicom BuildNES equ 1 Nestopia PC Engine BuildPCE equ 1 Ootake Super Nintendo (SNES) BuildSNS equ 1 Snes9x Vic 20 BuildVIC equ 1 Vice For these tutorials, I have provided a basic set of include files that will allow us to look at the technicalities of each platform and just worry about the workings of 6502 for now...
We will look at ALL of this code later, in the Platform specific series... but we can't do that until we understand 6502 itself!
The example shown to the right will load the A register with $69 (69 in hexadecimal)
We will then call the 'Monitor' function - which will show the state of the CPU registers to screen!
in this way, whatever the 6502 system you're learning and what emulator you're using, we'll be able to do things in a common way!
The example to the right is split into 3 parts:
The generic header - this will set up the system to a text screen
The program - this is where we do our work
The generic footer - The functions and resources needed for the example to work
It's important to notice all the commands are inset by one tab ... otherwise the Assembler will interpret them as labels.

The sample scripts provided with these tutorials will allow us to just look at the commands for the time being... we'll look at the contents of the Header+Footer in another series...

Of course if you want to do everything yourself that's cool... We're lerning the fundamentals of the 6502 - and they will work on any system with that processor... but you'll need to have some other kind of debugger/monitor or other way to view the results of the commands if you're going it alone!... Good luck!

Registers and Numbers

The 6502 has 3 main registers...
A is known as the Accumulator - we use it for all our maths
X and Y are our other 2 registers... we can use them as loop counters, temporary stores, and for special address modes... but we'll look at that later!
Lets learn our first commands... LDA stands for LoaD A... it sets A to a value... we can also do LDX or LDY to load X or Y registers!
Take a look at the example to the right... we're going to load A, X and Y... but notice... we're going to load them in different ways... A will be loaded with #$69... X will be loaded with #69... and Y will be loaded with 69... what will the difference be?? Well here's the result... the values are shown in Hex...
so A=69...  because specifying #$69 tells the assembler to use a HEX VALUE
but X=45...  this is because without the $ the assembler used a Decimal value (45 hex = 69 decimal)
Y=0... why? well when we don't use a # the assembler gets the memory address.... so we read from memory address decimal 00069!... of course we can do $69 or $0069 to read from address hex 0069 too!
So  #$xx = hex value .... #xx = decimal value .... and xx means read from address !

If you forget the # you're code is going to malfunction - as the assembler will use an address rather than a fixed value!

It's an easy mistake to make, and it'll mean your code won't work... so make sure you ALWAYS put a # at the start of fixed values!... or you WILL regret it!

Here are all the 6502 Assembler ways of representing values, and how they will be treated.

Prefix Example Z80 equivalent Meaning # #16384 16384 Decimal Number #% #%00001111 %00001111 Binary Number #$ #$4000 &4000 Hexadecimal number #' #'a 'a' ascii value 12345 (16384) decimal memory address $ $4000 (&4000) Hexadecimal memory address

We've been using this JSR command... but what does it do?
Well JSR jumps to a subroutine... in this case JSR monitor will run the 'monitor' debugging subroutine... when the subroutine is done, the processor runs the next command
In this case that command is 'JMP *' which tricks the 6502 into an infinite loop!
JSR in 6502 is the equivalent of GOSUB in basic or CALL in z80.... we'll look at how to make our own subroutine in a later lesson!

JMP is a jump command ... and * is a special command that means 'the current line' to the assembler... so 'JMP *' means jump to this line...

This causes the 6502 to jump back to the start of the line... so it ends up running the jump command forever!... it's an easy way to stop the program for testing!

The 6502 is a cut down version of the 6800... and would you believe it, one of the things they removed was the ADD and SUBtract commands!... so how can we do maths? well they did leave us some other commands... ADC and SBC ... these add and subtract a value plus the 'Carry'....

The Carry is a single bit which is the overflow from a previous calculation... you see, in 8 bit maths you can't go over 255... so if you set A=255, then add 1... then A will become Zero, but the Carry will be 1... effectively the Carry is the 9th bit!

Don't worry if you don't understand that now... the important thing is we need to deal with the carry before we try to add or subtract with ADC and SBC!

Note... there is no way to add or subtract with X or Y... you have to store to memory, and use a command like ADC $0013.... which would ADD the 8 bit value in memory address $0013

In this example, we're going to set A to Hex 15... then we'll show it by calling the Monitor
then we'll add 1... and show it again with the monitor
then we'll subtract 1... and show it again with the monitor
We don't want the Carry affecting things so we have to CL ear the C arry with CLC before the ADC command...
However strangely if we don't want the Carry to affect subtraction, we have to SE t the C arry with SEC ... before the SBC command - this is the opposite of the z80 command, but it's just the way the 6502 does things! Here is the result... you can see we go from 15, to 16, then back to 15!

We know how to set all the registers, but what if we have a value in one register, and we want to transfer it to another...
Well, we can use TAX and TAY to T ransfer A to X ...or T ransfer A to Y !
We can also use TXA or TYA to T ransfer X to A ... or T ransfer Y to A !
What if we want to transfer X to Y ? (or Y to X) ... well we can't directly, so we'd have to do TXA ... then  TAY You can see the result here... First we set A to $25 and Y to $34 - the result is shown on the first line
Then we transfer A to X... and Y to A... the result is shown on the second line.

Storing back to memory!

Remember we learned that using LDA with a number without a # means it will load from that numbered address? - so LDA $13 will LoaD A from hex address $0013?
Well we can also STore A with the STA command!... we can also STore X with STX , or Store Y with STY !
In this example we'll use STA to store some values to memory addresses $0011 and $0012
We'll then set the Accumulator to $13 and add these two memory addresses to the accumulator.... finally we'll use STA again to store the result to memory address $0013
When it comes to showing the result, we'll use another debugging subroutine I wrote called MemDump ... this will dump a few lines of data to the screen... in this case we'll show 3 lines (of 8 bytes) from memory address $0000-$0018... In this example, we'll show the memory before, and after we do the writes.
* Warning * If you're not using my sample code, these commands may overwrite system variables - and cause something strange to happen! Here's the result of the programm running... you can see the bytes $11, $22 and $66 were written... these are the two values stored at the start... and then the result of these two added to the $33 loaded into the accumulator
Want to try something else?? Why not change CLC to SEC and  ADC to SBC... and see what happens!

The first 256 bytes of memory $0000-$00FF are special on the 6502... in fact there's a lot we're not mentioning about reading and writing memory... but it's coming soom!

Also the memory from $0100-$01FF is also special... it's used by the stack!... don't know what that is? don't worry... we'll come to that!

Be Careful writing to memory on different systems... This example may not work write on some systems...

The PC-Engine is weird... unlike every 6502... the range $0000-$01FF is NOT memory... that area is at $2000-$21FF

Why? because it's not actually a 6502... its a HuC6280... it's almost the same as a 6502... but it has some extras and weirdness!

Lesson 2 - Addressing modes on the 6502

The 6502 has very few registers - but it makes up for this with a mind boggling number of addressing modes!

You won't need them all at first, but you should at least understand what they all do - lets see some examples of how they work!

Lets try them all out with some simple examples!

Prepearation...

In order to run these examples we're going to need to set up some areas of memory, by filling them with test values.
The code to the right will do the work (via a Function called LDIR - which copies memory areas)... don't worry how it works for now, it's too complex at this time! Here is the rest of the Chunk copying code, and the data copied... again, you don't need to worry about this for now. Prepearation... the result... Here is the important bit... THIS is the data as it appears in memory when the program runs... you may want to refer back to this if you wish!

Note: These tutorials will not work on all systems... for example most will not work on the PC engine, because the zero page is not at &0000!

They may also not work on the NES or SNES, because the &2000 area has a special purpose on those systems.

They have all been tested on the BBC.... but don't worry... the theory shown here is based on the principals of the 6502 - so will work on ANY 6502 based system!

We're all set up now... lets try out all the addressing options... we'll look at the theory, and an example program... then we'll see the result in the registers in a screenshot from the BBC version
We'll be reading in all these examples... but many of the commands can be used for other commands.. please see the Cheatsheet for more details.

1.Relative Addressing

Relative Addressing is where execution (the program counter) jumps to a position relative to the current address - it can be 127 bytes after the calling line, or 128 bytes before....

This means the code will be 'relocatable' - we can move it in memory and it will still work, but we can't jump more than 128 bytes!

There are all kinds of 'Branch' commands... here we've used 'Branch if Carry Clear'... we'll look at the others in a later lesson

BCC ALWAYS takes a fixed number (not an address), so we don't have to use # with BCC in vasm!... that said, we can just use labels (names that appear at the far left, and let the assembler work out the maths.

Take a look at the example to the right... there are 3 Monitor commands... but only 2 show on the screen... this is because the BCC skips over one
The "Program Counter" (shown as P) stores the byte of the end of the last command.... A "JSR Monitor" takes 3 bytes, "BCC 3" takes 2... hopefully the numbers the program counter shows will now make sense if you add up the commands!

2.Accumulator Addressing

Accumulator addressing sounds more complex than it is!
Effectively it's a command with no parameters - it just changes the accumulator in some way.... For Example LSR shifts the bits to the left... don't worry if you don't understand it, we'll look at it later!

Again, Immediate sound scary... but it's really easy... it's just a simple number in the code, specified with a #
As we've already learned... we can use # followed by $ to sepcify a hexadecimal number. In this example we will add Hex 10 and Hex 20... the result is obviously 30!
Why not try using different numbers,remove the $ to stop using hexadecimal..., or SBC... don't forget to change CLC to SEC if you do! 4.Zero Page Addressing The Zero Page is the 6502's special trick... addresses between $0000 and $00FF are called the 'Zero Page'... these can be stored as a single byte... so $FF would refer to address $00FF
Because the address is stored as a single byte - it's fast, and the Zero page can do things that other addresses cannot!
The 6502 uses this 'zero page' like a bank of 255 registers - allowing the 6502 with it's just 3 registers to do the things the Z80 did with over a dozen! In this example we'll load from zero page address $80.... note that if we did LDA #$80 then we would load the Value $80 not from the address...
This is important - you don't want to make that mistake (too often!)

The Zero Page (Sometimes called the Direct Page - usually when it's not at $0000) is effectively the 'tepmporary store' for all the data we can't get into the A,X and Y registers...

We can use different numbered addresses for different purposes, but many may be used by the machines firmware!

5. Zero Page Indexed X (or Y with LDX / STX) Addressing When we specify ,X or ,Y after an address it becomes an offset... the register is added to the address in the zero page... and the value is retrieved from the resulting address...
Note - you typically have to use X for this addressing mode... however LDX and STX are as special case, and we can use Y because we can't use X if it's the source or destination of the command
Note... LDA $20,Y is not a valid command... however the assembler will covert it to LDA $0020,Y which IS... but it takes an extra byte, so is not as efficient! As you can see here we're using the Zero Page, and X and Y register....
take a look at the values we wrote to the Zero Page at the start, and try changing X,Y and the source location ($80) to other values.

6. Absolute Addressing

Of course we can't always read and write in the zero page... we'll want to specify the whole address... this takes an extra byte - so the command will be 3 bytes total and is slower, but we can get data from the whole 64k range ($0000-$FFFF) Absolute addressing is good for variables we're not storing in the zero page (often most of the Zero page is used by the firmware!)... but isn't very good for reading in lots of data (like sprite images)... for that we want indirect addressing - which we'll look at soon!

7. Absolute Indexed Addressing With X,Y

When we want to read from multiple addresses, we can used Indexed addressing... this adds X or Y to an address - so we can change X/Y to read in from a range using a Loop!... we'll learn how to do a loop very soon!
$xxxx,Y can be used with many commands, but $xxxx,X has more options... check out the cheatsheet for more info! Changing X and Y allow you to change the source address without changing the LDA line.... we'll learn how to do this in loops and functions later.

8. Absolute Indirect

We can directly read a 16-bit value from another 16-bit address ($0000-$FFFF) In one special... the JuMP command (for all other cases we need to use the zero page.
This can be used to reprogram parts of your progam - allowing alternate routines to be 'switched' in. In this example we use ($2000)... this loads in two bytes $1B1A and then jumps to that address (sets the PC to 1B1A)...
Our setup put a "JSR MONITOR" at this address... so we see the contents of the registers... notice P (the program counter) is $1B1C... the last byte of the 3 byte "JSR MONITOR" command

9. Preindexed Indirect Addressing with X 

Pre-inxexed Indirect with X regsiter uses the ZeroPage... X is added to the ZeroPage.... the two consecutive bytes are read in from the zero page, and these are used as an address... a byte is read from that address... Note... the data is stored in 'Little Endian' format... meaning the lower value byte comes first
This is all very cofusing!... but think of it like this... two bytes of the zeropage are a 'temporary address' pointing to the actual data we will read
We can use these to simulate 'Z80 registers'...  by setting one as an L register for the low byte, and the next as the H register for the high byte....
This is how we get around the 6502's lack of registers!... don't worry about it if you don't understand yet... we'll see this a lot later! In this example we've got X set to 1... so we end up loading a byte from the address made up of bytes at $0081 and $0082 - remember they are in reverse order because it's little endian!
we then show the result to screen.... of course setting X to 0... and changing $80 to $81 would have the same effect.

10. Postindexed Indirect Addressing with Y 

Post-Indexed with the Y register also use the Zero Page... two concecutive bytes are read in from the Zero page to make an address... but the Y register is then added to THAT address... and the final value is read from the resulting address.
With this option, Effectively, if we store an address in the Zero page... we can use Y as a counter and read from consecutive addresses... we can use this in a loop - we'll learn how to do that later Y is 2 in this example, so 2 is added to the address in ZeroPage ($0080-$0081)... if we change Y then the final address will change by the same amount

11. Indirect Addressing (65c02 only)

This is a special mode only available on 65c02 used by the Lynx, Snes, PcEngine and Apple II....
Effectively it's the same as Preindexed when X=0... or PostIndexed when Y=0... this is how we can simulate this addressing mode if we need to do this on the other machines!
It uses a pair of bytes in the Zero page as an address, and uses that address for the result It would be nice to have this mode on the other CPU's, but we don't... however we can simulate it!
to fake it on other machines we set X=0 then use LDA ($81,X)
or we set Y=0 and then use LDA ($81),Y

You won't see much '65c02 only' code in these tutorials - so all the code will work on all systems, we only use the basic 6502 commands

Of course you're free to use them if you wish, just remember - it will mean you can't port your code to another system as easily!

Lesson 3 - Loops and Conditions
We've had a breif introduction to 6502, and now we understand the Addressing modes we can look properly at 6502, lets take a look at some more commands, an how to do 'IF Then' type condions and Loops!

We've been cheating a little, we've overlooked a few important commands - they're hidden in the header, but we really need to know them!... before we start the proper lesson, lets look at them now!
We're going to need to know ALL the details of assembly to create a working program, and something have been hidden until now! but we need to ensure we know everything.

Because we're compiling to a 8-bit cpu with a 16-bit address bus, our compiled code filles maps to a fixed address within the memory space... this is important, because while branch commands like BCC are an 'offset'... JMP commands will 'Jump' to a specific numbered address
to the right, you can see how the code will compile - this is the 'Listing.txt' file, showing the source code and the resulting binary output.
The SEI command is compiled to the byte $78 - this is the command as the CPU sees it... because of the ORG command, the code is compiled to the address $ 0200 ...

We also have a Label ...  Labels must be at the far left of the screen... all other commands must be inset
n this example, the label will be defined as address $0200 - so if we use it in a Jump command (hex $4C) , it will be compiled to that address (in reverse endian - so $0200 becomes $00 $02)

SEI - Disabling interrupts

Interrupts are where the CPU does other tasks whenever it wants!
For simplicity at this stage, we want to stop that, so we use SEI to "Set the Interrupt Mask"
Don't worry about interrupts yet, we'll look at them later... so for now we just need to know how to turn them off

Symbol definitions

Symbols are similar to labels... they allow us to give 'name' (like TestSym) a 'Value' ... rather than using the value later, we can just use the symbol... Using symbols makes it easy for us to program, as we can use explainatory text rather than meaningless numbers.
the assembler will convert the symbol name to its original value... we just use EQU to define the definition... in the example once assembled LDA converts to byte $A5... and TestSym has a value of $69
In VASM, like labels, symbol definitions  must be at the far left of the screen

INC and DEC

There will be frequent times when we need to increase and decrease values by just 1
For the X or Y registers we can do this with INX and DEX
We can increase values in the ZeroPage by using INC $01 or DEC $01
rather annoyingly there is no INC or DEC command on the 6502... so we have to simulate it, by clearing the carry, and adding one (CLC, ADC #1) Here you can see the results of the program...
The first thee lines show the status of the registers at each stage.... and we can see how A,X and Y are affected by each stage of the program
The lower half shows the zero page - and we can see how $01 goes up and down as we do INC and DEC commands

Branch on condition

Branches allow us to do things depending on a condition... we can use this to create a loop!
Because we don't have a DEC command for the accumulator, it's often easier to use X or Y as a loop counter.
if we use DEX to decrement the counter, and BNE will jump back until the counter reaches zero... note that BNE needs to be immediately after the decrement command as other commands may alter the Z flag

There are a wide variety of Branch commands for different condition codes.

Command Meaning Literal Meaning Description BCC Branch if Carry Clear flag C=1 Is there any carry caused by last command?* BCS Branch if Carry Set flag C=0 Is there any carry caused by last command?* BEQ Branch if Equal flag Z=1 Is the result of the last command zero? BMI Branch if Minus flag S=1 Is the result of the last command <128 BNE Branch if Not Equal flag Z=0 Is the result of the last command zero? BPL Branch if Plus flag S=0 Is the result of the last command >=128 BVC Branch if Overflow Clear flag V=0 Is there any overflow caused by there last command?* BVS Branch if Overflow Set flag V=1 Is there any overflow caused by there last command?*

If a previous addition command caused a value over 255 then Carry will be set... Overflow is a bit odd... it's affected if Addition/Subtraction goes over the 128 boundary (if it changes from positive to negative) it's also set by BIT commands

Comparing to another value with CMP, CPX and CPY

If you don't want to see if a register is zero, you can compare to a different value with CMP... then perform one of the commands.... effectively, CMP 'simulates' a subtraction

Basic command Comparison 6502 command Z80 equivalent 68000 equivalent if Val2>=Val1 then goto label >= BCS label JP NC,label BGE label if Val2<Val1 then goto label < BCC label JP C,label BLT label if Val2=Val1 then goto label = BEQ label JP Z,label BEQ label if Val2<>Val1 then goto label <> BNE label JP NZ,label BNE  label

Conditional Jumping far away with JMP, or calling a subroutine with JSR

Branch commands are pretty limited, they can only jump 128 bytes away, if you try to jump further you will get an error If you need to jump further, or you want to use JSR with a condition you have to do things backwards!.... jump OVER the JSR or JMP command if the condition is NOT met
For example... if you want to call the Monitor if X=2... then you have to use a branch command to jump OVER the call if X is not 2... The result is that the monitor is called only when X=2... we've faked a 'Jump to SubRoutine on Equal' command... we can also do the same with a JMP to get further than 128 bytes away!

Using BVC to simulate BRA

JMP jumps to a specific memory address, where as BEQ and other branch commands jump to a relative position...
There may be cases where you want to write code that can be relocated... copied to a new memory address and still executable... JMP will not work in this case, but branch will...
the 65c02 has a BRA command for this purpose (branch always)... but the 6502 does not... we can however simulate it by clearing the rarely used overflow with CLV, then using BVC
Don't worry if you don't see any reason to do this - you may never need to! if you don't know why you'd need relocatable code - then you don't need it!

Multiple conditions for a Case statement

It's important to understand that ALL other languages convert to assembly... so anything Basic or C++ can do can be done in ASM!
We can chain multiple branches together to create 'If Then ElseIf' commands or even create 'Case' Statements in assembly, just by chaining multiple branch commands together. The result will be the program will branch out to each of the subsections depending on X Through a combination of conditions we can do any condition in assembly that C++ or Basic can do... that's because those languages compile DOWN to assembly...
That said, it will take a lot more work in assembly! Lesson 4 - Stacks and Math

Now we know how to do conditions, jumping and the other basics, it's time to look at some more advanced commands and principles of Assembly..

Lets take a look!

Stack Attack! 'Stacks' in assembly are like an 'In tray' for temporary storage...
Imagine we have an In-Tray... we can put items in it, but only ever take the top item off... we can store lots of paper - but have to take it off in the same order we put it on!... this is what a stack does!
If we want to temporarily store a register - we can put it's value on the top of the stack... but we have to take them off in the same order...
The stack will appear in memory, and the stack pointer goes DOWN with each push on the stack... so if it starts at $01FF and we push 1 byte, it will point to $01FE

Push me - Pull me!

on the Z80 we have Push and Pop, but on the 6502 it's Push and Pull!
We PUSH values onto the top of the stack to back them up, and PULL them off!
Our 6502 has 4 registers we may want put onto the stack A, X, Y and the 'Flags' ... unfortunately the basic 6502 can only directly do A and the Flags - so we will have to Transfer X/Y to A first ... but the 65C0C can do it directly.
When it comes to setting the 'Stack pointer' we have to do it via the X register - Remember, the stack HAS to be between $0100 and $01FF on the 6502 Action 6502
command 65C02
Command Action 6502
Command 6502
Command Push A PHA PHA Pull A PLA PLA Push X TXA
PHA PHX Pull X PLA
TAX PLX Push Y TYA
PHA PHY Pull Y PLA
TAY PLY Push Flags PHP PHP Pull Flags PLP PLP Set SP to X TXS TXS Set X to SP TSX TSX Let's try out the stack!
We're going to set A,X and Y to various values, and push them onto the stack,
Because we can't do this directly for X and Y, we'll have to transfer them to A first
Once we've done that, we'll show the contents of the stack...
We'll then clear all the registers - and pull them from the stack - it's important we pull them in the same order!
Finally we'll show all the register contents We can see the 3 bytes at the top of the stack - remember the stack pointer goes down with each push, so they are backwards
Provided we restore them in the correct order - the registers are restored - even though we cleared them before Subroutines are sections of code that will be executed, and then execution will resume after they complete
On the 6502 we call a sub with JSR (Jump SubRoutine).... and  the last command of the sub is RTS (ReTurn from Subroutine)
if you're familiar with basic JSR is the equivalent of GOSUB... and RTS is the equivalent of RETURN
We're going to do a test here... we'll show the stack to the screen... first we'll push the flags onto the stack,
Then we're going to use JSR to jumpt to subroutine StackTest.... we'll show the stack again... and for reference, we'll also see the address of 'ReturnPos'
Then we'll return to the main program and show the stack again... what will happen? The flags are pushed onto the stack first... Next we can see the 'Return address' , that was pushed onto the stack by the JSR command
Effectively JSR pushes the program counter onto the stack, and RTS pulls the Program Counter off the stack Because the JSR and RTS commands use the stack to maintain the program counter, it's important that the stack is the same when a subroutine ends as it was when it starterd... ne need to ensure we pull everything off the stack that we pushed on at the start... otherwise some 'other data' will be mistaken for the return address - and anything could happen!

Negative numbers in Assembly

Negative numbers in HEX are weird!... when we subtract 1 from 0 we get 255... this means 255 IS -1... in the same way, 254 is -2 and so on - meaning a 'Signed' byte can go from -128 to +127
The CPU doesn't 'Know ' whether it's working with signed or unsigned numbers - it all depends how we use the data...
The psuedocode for converting to positive to negative is to invert all the bits , and add one ... or subtract the value from zero of course! When we put a #-1 in the source, its converted to 255...
Because the numbers wrap around, adding 255 to a number decreases it by 1... so 255 IS -1
if we want to negate a number, we flip all the bits and add one... this converts 01 to $FF

Conditional Assembly

We learned about using Labels for Jumps, and Symbols for values before... but symbols have another use!
We can put IFDEF statements in our code, and have parts of the assembly only compile if a symbol is defined - or not defined with IFNDEF
It's important to understand, it's not the CPU doing ths, the assembler simply skips over the excluded code - so it never appears in the outputted binary!
This allows us to build multiple versions of a program from a single source, in fact it's how these tutorials support so many systems!
To disable a definition we can just rem it out with a semicolon ;  - we can even define symbols on the Vasm Command line! The output will of course be completely different depending on whether TestSymbol is defined or not. With TestSymbol Defined
Without TestSymbol Defined

Macros... for less typing!

Subroutines are great - but there's times they may be too slow (because of the JSR/RTS) .... and if you want to do things with the stack, they may not be possible.
Alternatively, we can use a Macro ... this is a chunk of code that we can give a simple name... then whenever we use that name - the assembler will insert the code... we can even use parameters in the macro.
Because the assembler does the work, it's faster than a call, but saves us typing all the commands... however it will make the code larger - so you will want to call to subroutines for big chunks of code where you can rather than use macros.

16 bits.. When 8 Bits aren't enough!

Unlike the Z80, we don't have pairs of registers which we can use for 16 bit commands,
the easiest solution to this is to use concecutive bytes of the Zero Page as a pair to make up a 16 bit 'Zero Page Register'
For ease of use, we'll use Symbols to define these with a name - and we'll mimic the Z80 register pairs... for example HL is High Low... but because the 6502 is little endian, L comes first in the zero page When it comes to Addition or Subtraction - we use the Carry flag...
The Carry flag stores the 'overflow' of an addition, or the 'borrow' of a subtraction.
By using two ADC we can add 16 bit (or more) numbers, and two SBC 's can do a 16 bit subtract When we want to use a 16 bit value, we have to split it into it's High byte, and it's Low byte
Forunately 6502 assemblers have us covered... we can use a > to calculate the high byte of a number, and < to calculate the low byte
Once we've set 16 bit pairs Z_DE and Z_HL, we can call the addition or subtraction function
Note: many of the 'Printchar' functions use the same 'Z Page' values... so we're using a special 'PrintHex' function that backs them up. Addition:
Subtraction:

There's no needs to stop at 16 bits, you can just keep doing ADC's to get up to 32 bits or more...

Of course it will be slower!... another option is 'floating point'... but that's a too complex to cover here!

These tutorials use Zero page registers to mimic the function of Z80 registers where the 6502 can't directly do the job... this is because the author of these tutorials started on the Z80, and found that the most logical way to do things...

Other Tutorials may do things differenty, and if you don't like this way of using the Zero page, you should probably follow another tutorial instead.

Mult/Div... Where's my Maths!

The Z80 and 6502 have something in common... they have no Multiply or Divide commands... yes, you read that right!
We can, however simulate them!... the simplest way to multiply is repeately add a value, or subtract one to divide...
There are faster ways of doing things - and we'll look at them later!
In our Multiply example we'll multiply A by X, and store the result in A
In our Divide example we'll Divide A by X, and store the successfull divisions in X, and the remainder in A You can see we've effected a simple Multiply and Divide command! Lesson 5 - Bits and Shifts
We've learned lots of maths commands, but we've still not covered the full range... this time lets take a look at how we can work with Bits on the 6502!

AND, OR and EOR!

There will be many times when we need to change some of the bits in a register, we have a range of commands to do this!

AND will return a  bit as 1 where the bits of both the accumulator and parameter are 1

OR will set a bit to 1 where the bit of either the accumulator or the parameter is 1

EOR is nothing to do with donkeys... it means Exclusive OR... it will invert the bits of the accumulator with the parameter - it's called XOR on the z80!

Effectively, when a bit is 1 - AND will keep it... OR will set it, and EOR will invert it

A summary of each command can be seen below:

Command Accumulator Parameter Result AND 1
0
1
0 1
1
0
0 1
0
0
0 ORA 1
0
1
0 1
1
0
0 1
1
1
0 EOR 1
0
1
0 1
1
0
0 0
1
1
0 Command lda #%10101010
eor #%11110000 lda #%10101010
and #%11110000 lda #%10101010
ora #%11110000 Result #% 0101 1010 #%1010 0000 #% 1111 1010 Meaning Invert the bits where the
mask bits are 1 return 1 where both bits are1 Return 1 when either bit is 1

In the Z80 tutorials, we saw a visual representation of how these commands changed the bits - it may help you understand each command.

Sample EOR %11110000
Invert Bits that are 1 AND %11110000
Keep Bits that are 1 ORA %11110000
Set Bits that are 1 Lets try these commands on the 6502!
We'll use a test bit pattern, and try each command with the same %11110000 parameter,
We're using a 'MontiorBits' function, which will show the contents of the Accumulators bits to screen! The bits of the test pattern will be altered in each case according to the logical command!

Rotating and shifting bits with ROL,ROR, ASL and LSR

There will be many times when we want to shift bits around... If we shift all the bits in a byte left, we'll effectively double the number - if we shift them right, we'll halve it

We may want to use 3 bits from the middle of a byte or word as a 'lookup' - and we'll need to get them in the right position...

You may not immediately see the need for bit shifting - but as you program, you'll come across many times you need to do it...

One very important use of ASL/LSR is for halving and doubling numbers... our CPU has no multiply or divide commands, but effectively it can quickly do x2 or /2... and you want to try to take advantage of this when designing your code!

The 6502 has 2 options - shift a bits within the Accumulator  using ASL or LSR - which will fill any new bits with 0 and lose any bits pushed out of the accumulator,

or 'Rotate it through the carry flag' with ROL and ROR... where the carry is put into the new bit, and any bits pushed out go into the carry flag

Command Left Right ROtate ROL ROR Arithmatic Shift /
Logical Shift ASL LSR We're going to test the shifting commands... we'll use a new testing function 'MonitorBitsC' will show the Accumulator and Carry flag.
We'll set the accumulator to %10111000, and we'll clear the carry flag...
Then we'll see what happens when we use each of the rotate commands 9 times! So what does each command do?
Well ROL rotates all the bits Left, the carry ends up in Bit 0 - and what WAS in Bit 7 ends up in the carry.
ROR is the opposite... it rotates all the bits Right, the carry ends up in Bit 7 - and what WAS in Bit 0 ends up in the carry.
ASL shifts all the bits left - but Bit 0 is zero - and the what was in Bit 7 is lost
LSR is the opposite, it shifts all the bits right - but Bit 7 is zero - and the what was in Bit 0 is lost ROL:
ROR:
ASL:
LSR:
The 6502 doesn't have as many bit shift options as the Z80... but we can 'fake' others!.
If we want to shift 1's into the empty bits we can just set the carry with SEC before the rotate command,
If we want to rotate the 8 bits in the accumulator without the carry... we can back up A with PHA, do the rotate, then restore A with PLA, and do another rotate Now we're able to set the new bits to a 1, or able to rotate the bits within A
There's other ways to do this, and other combinations of commands to do things like swap nibbles... seehere SEC -ROL
SEC-ROR
PHA-ROL-PLA-ROL

PHA-ROR-PLA-ROR

There's lots of commands we'd like to have that are 'missing' on the 6502 - and this is just one possible solution

Seefor more examples of combinations of commands to effect the result you want.

We could use the AND command, but that will change the accumulator - and we may want it to stay the same... for this we have the BIT command

BIT has the same effect as AND on the Z flag - but doesn't change the Accumulator... unlike AND, we have to use a memory address as the parameter... so we'll define a set of bitmasks...

Because the BIT command needs to work with an address, we need to define some bitmasks...
To define a byte of data in our program code we use DB - then we specify the value for the byte... we're using % and defining the definitions in bits
We're giving each of these a label, so we can use them easilly later. We can use the BIT command with a label pointing to one of these defined bytes, and then use BNE or BEQ to branch depending on if the bit was Zero or not...
Note, the Accumulator is unchanged when we do this We'll branch and show a B if the bit is Zero... or an A if the bit is One
Hint: Try changing the TBit1 to a TBit0 in the example code! Specifying Addresses in this way will use 3 bytes per command - which is wasteful - if possible, it would be better to store these bitmasks in the Zero page, so we only use 2 bytes per command if we can.

Whatever bit you test, two other flags are set at the same time....as well as the Z flag being set to the tested bit, N flag is set to bit 7 , and the V flag is set to bit 6

So you can branch on conditions relating to bit 7 and 6 without any more testing commands!

NOP - Slacking in 8 bits!

NOP (No OPeration)  is a strange command... it does absolutely nothing!
Why would we want to use it? well it's handy for a short delay - and if we do something called 'Self Modifying code' (code that rewrites itself) it can be useful for disabling commands The more NOPs we add, the slower the screen will fill

Lots of NOP commands aren't really a good way of slowing things down - It's far better to nest loops to slow things down or use some kind of firmware function...

NOP's are more useful for self modifying code - we'll learn about that next time!