26. ASM Basics

Intro to 6502 ASM, with the ca65 assembler. Links for reference…

http://www.6502.org/tutorials/6502opcodes.html

http://www.obelisk.me.uk/6502/reference.html

I’ve had a lot of questions about 6502 ASM. One of the features of cc65 is the ca65 assembler, which is a very good one. You can write any, or all, your functions in assembly. But, it would help if you knew how 6502 ASM works…so I’m going to write a few tutorials. All my examples will be ca65 specific, but it might be useful for users of other assemblers. Note, I will usually use $ to indicate hex numbers.

The 6502 has 3 registers. A (the accumulator) and X and Y (index registers). A is for most purposes and calculations. X and Y are for counting loops or indexing arrays.

The NES has 256 zeropage RAM addresses (that is addresses 0 – 0xff) and 1792 non-zeropage RAM addresses (addresses 0x100 – 0x7ff). Some games also have an additional RAM chip on the cartridge, usually mapped to 0x6000-0x7fff. If powered by a battery, it SAVES the game. We prefer zeropage variables to the others, because they only need 1 byte of ROM, to refer to it, rather than 2. Also, they are faster operations.

Let’s go over the syntax of the most common opcode, LDA (load the A register).

LDA 10 (will load the A register from the zero page address $000A). $A is hexadecimal for decimal 10.

LDA $10 (will load the A register from the zero page address $0010. $ means hexadecimal number.)

LDA #10 (will load the A register with the value 10. # means immediate mode. We want a value and not a RAM address.)

LDA #$10 (will load the A register with the value 16, which is the same as $10. Again, the # means it’s a value, not an address.)

Declaring constants.

zip = 0
FOO = $3f
LIVES = $03e3

When used in assembly code, the assembler will replace the symbol with the value you define.

Examples:

-> here means ‘assembles into’
the left-most part is what you will be typing

LDA #zip -> LDA #0   load A with value 0, the '#' means value, not address
LDA zip  -> LDA 0  load A from the 8-bit address $00
LDA #FOO -> LDA #$3f load A with the value $3f
LDA FOO  -> LDA $3f  load A from the 8-bit address $3f
LDA LIVES -> LDA $03e3 load A from the 16-bit address $03e3
LDA # LDA #$e3 load A with the value (lower byte of $03e3 = $e3)
LDA #>LIVES -> LDA #$03 load A with the value (upper byte of $03e3 = $03)

< gets the lower byte of a 2 byte expression > gets the upper byte of a 2 byte expression

LDA #LIVES -> produces an error. The assembler was expecting an 8-bit number.

Declaring variables.

.segment "ZEROPAGE"
foo: .res 1
bar: .res 2

foo will be reserved 1 byte at address $00, and bar will be reserved 2 bytes at addresses $01 and $02.

LDA foo  -> LDA $00 load A from the 8-bit address $00
LDA bar  -> LDA $01 load A from the 8-bit address $01
LDA bar+1  -> LDA $02 load A from the 8-bit address ($01 + 1 = $02)

Note that the bar+1 here is a “compile time constant”. The assembler knows the value of bar and will add one to it at compile time. This will still execute as a standard LDA from a zero page address.

.segment  "BSS"
fooz: .res 1
baz: .res 2

As I described above, I’m defining the BSS section in the .cfg file as being from $300-$3ff. Therefore, the assembler will reserve 1 byte for fooz at $0300 and 2 bytes for baz at $0301 and $0302.

LDA fooz  -> LDA $0300 load A from the 16-bit address $0300
LDA baz  -> LDA $0301 load A from the 16-bit address $0301
LDA baz+1  -> LDA $0302 load A from the 16-bit address ($0301 + 1 = $0302)

Importantly, we don’t need to know what value is reserved when writing code. The assembler will keep track of the values and addresses of every label, you just have to reference them in your code using the labels/variable name.
* constants and variables should be defined at the very top of the ASM page.

Referencing ROM addresses in code, using labels.

.segment  "CODE"
LDA Table1
LDA Table1+1
...
Table1:
.byte $01, $02

(note, you could also put Table1 in the “RODATA” segment, if you like)

Let’s say that the assembler has calculated that Table1 will be at address $8050. This will assemble into…

LDA $8050 load A from the 16-bit address $8050
LDA $8051 load A from the 16-bit address $8051

when the program is RUNNING, the first line will load A with the value #01 and the second line will load A with the value #02…because those are the values in the ROM at 8050 and 8051.

OK, now that we understand how the labels work, let’s do some code…using C examples, and how to do it in ASM. There are 3 registers in the 6502. A, X, and Y.

foo = 3;

LDA #3  	load A with value 3
STA foo  	store A at address foo

or we could have used the other registers…

LDX #3  	load X with value 3
STX foo  	store X at address foo
or
LDY #3  	load Y with value 3
STY foo  	store Y at address foo

There is no difference which register you use for this kind of thing.
.

bar = $31f; //a 16-bit value

From working with cc65, I now have a habit of using A for low bytes and X for high bytes (as the cc65 compiler tends to do)…

LDA #$1f 	load A with the value $1f
LDX #$03 	load X with the value 3
STA bar  	store A to address bar
STX bar+1 	store X to the address bar+1
we could also have done...
LDA #$1f 	load A with the value $1f
STA bar  	store A to address bar
LDA #$03 	load A with the value 3
sta bar+1 	store A to the address bar+1

baz = bar; //a 16-bit value

LDA bar  	load A from the address bar
LDX bar+1 	load X from the address bar+1
STA baz  	store A to the addres baz
STX baz+1 	store X to the address baz+1
again, we could have done...
LDA bar  	load A from the address bar
STA baz  	store A to the addres baz
LDA bar+1 	load A from the address bar+1
STA baz+1 	store A to the addres baz+1

Next thing…increment / decrement

foo++;

INC foo  add 1 to the value stored at foo

foo- -;

DEC foo  subtract 1 from the value stored at foo

you can also increment and decrement the X and Y registers

INX  add 1 to the X register
INY  add 1 to the Y register
DEX  subtract 1 from the X register
DEY  subtract 1 from the Y register

You will have to use adding/subtraction to ++ or – – the A register.
Which brings us to simple math…in fact very very simple math. The 6502 can only do addition and subtraction, and bit-shift multiplication. And, ONLY the A register can do math or bit-shifting.

Adding is always done ‘with carry’. The 6502 has certain FLAGS to assist math, and for doing comparisons. If the result of addition is > 255, then it sets the carry flag – in case you are doing 16-bit math (or more). If the result of addition is <= 255, the the carry flag is reset to zero. But, it always adds A + value + carry flag. Therefore, we must ‘clear the carry flag’ before addition. Here’s an example…

A reg. + value + carry flag = 
result now in A // carry flag
4+4+0  = A = 8, carry = 0
4+4+1  = A = 9, carry = 0
255+4+0  = A = 3, carry = 1
255+4+1 = A = 4, carry = 1

foo = fooz + 1; //8-bit only

LDA fooz 	load A from address fooz
CLC  		clear the carry flag
ADC #1  	add w carry A + value 1, the result is now in A
STA foo  	store A to the address foo

or, reverse them, get the same result…
foo = 1 + fooz; //8-bit only

LDA #1  	load A with value 1
CLC  		clear the carry flag
ADC fooz 	add w carry A + value at address fooz, result now in A
STA foo  	store A to the address foo

Let’s do a 16-bit example.

bar = baz + $315; 16-bit values

LDA baz  	load A from address baz
CLC  		clear the carry flag
ADC #$15 	add w carry A + value $15 (the low byte)
  if the result is > 255, the carry flag will be set, else reset to zero
STA bar  	store A to the address bar
LDA baz+1 	load A from the address baz+1
  ...notice, we don't clear the carry flag, we are using the
  carry flag result of the previous addition as part of this addition
ADC #$03 	add w carry A + value $03 (the high byte)
STA bar+1 	store A to the address bar+1

And, some subtraction. Like the ADC, subtraction always uses the carry flag, but in reverse. It’s called Subtract with Carry. You need to SET the carry flag before a SBC operation. If the result of subtraction underflows below 0, it will reset the carry flag to zero. Else, it will set the carry flag. Again, this is in case you want to do 16-bit (or more) math.

(on a side note the carry flag here works the opposite as is does on most other microprocessors, where the carry flag works as a borrow.)

Here’s some examples…
! = NOT…ie, the opposite

A reg. - value - !carry flag = 
result now in A // new carry flag status
8-4-!1  = A = 4 // carry = 1
8-4-!0  = A = 3 // carry = 1
4-5-!1  = A = 255 // carry = 0
4-5-!0  = A = 254 // carry = 0

foo = fooz – 1; //8-bit only

LDA fooz 	load A from address fooz
SEC  		set the carry flag
SBC #1  	subtract value 1 from A, result is now in A
STA foo  	store A to address foo

And the reverse, which is a different thing altogether…

foo = 1 – fooz; //8-bit only

LDA #1  	load A with value 1
SEC  		set the carry flag
SBC fooz 	subtract value at address fooz from A, result is now in A
STA foo  	store A to address foo

And a 16-bit example…
bar = baz – $315; //16-bit numbers

LDA baz  	load A from address baz
SEC  		set the carry flag
SBC #$15 	subtract value $15 from A, result is now in A
STA bar  	store A to address bar
LDA baz+1 	load A from addres baz+1
  ...notice, we DON'T set the carry flag. We are using the result
  of the last math to set/reset the carry flag.
SBC #$03 	subtract value 3 from A (and subtract !carry), result now in A
STA bar+1 	store A to the address bar+1

.

Stay tuned for many more ASM lessons to come.

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