I'm trying to learn more about how the 6502 processor works but am stuck on how subtraction is done.

Here is my understanding of addition:

---------------------------------------------------------------------

[Addition with 2 bytes]

First number:
310 decimal / $136 / 0000 0001 0011 0110
$03A0 = 0011 0110 - low byte
$03A1 = 0000 0001 - hight byte

Second number:
525 deciaml / $20D / 0000 0010 0000 1101
$03B0 = 0000 1101 - low byte
$03B1 = 0000 0010 - byte

Program:
CLC ;Clear carry flag
LDA $03A0 ;Load low byte of first number into accumulator
ADC $03B0 ;Add low byte of second number to low byte of first number with carry
STA $03C0 ;Store result in memory
LDA $03A1 ;Load high byte of second number into accumulator
ADC $03B1 ;Add high byte of second number to high byte of first number with carry
STA $03C1 ;Store final result in memory

Result:
$03C0 = 0100 0011 - Low byte
$03C1 = 0000 0011 - High byte

[How it is done]
__111 1 Add with carry
_0011 0110 ($03A0 Location) - First number low byte
+0000 1101 ($03B0 Location) - Second number low byte
--------------
_0100 0011 ($03C0 Location) - Low byte result

_0000 0001 ($03A1 Location) - First number high byte
+0000 0010 ($03B1 Location) - Second number high byte
--------------
_0000 0011 ($03C1 Location) - High byte result

Result:
835 decimal / $343 / 0000 0011 0100 0011

---------------------------------------------------------------------

I know the carry flag is set before subraction but I can't see why. So can someone explain how subraction is done with the 6502. Perhaps using an example like the one above.

John,

I'll do my best to explain it as I understand it. My examples will use a 4 bit number for simplicity.

In order to do a subtraction in hardware, its usually easier to take the two's compliment of the second number and then add the two numbers together. In that way, you only need a hardware adder to do both addition and subtraction.

Now, to take the two's compliment of a number, you invert the bits and then add 1.

Example:
0101 = 5
1010 = -6 (invert the bits, or one's compliment, or EOR $FF)
1011 = -5 (add 1)

Now, instead of adding the 1 seperately, why not just set the Carry and do an ADC command, saving an extra cycle!

Here are two examples:


That how I understand it. I hope its clear to you.

Daryl
http://65c02.tripod.com/

Thanks for the explanation...Now I understand.