Greater-Than or Less-Than ASM?

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Main - ROM Hacking - Greater-Than or Less-Than ASM?

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RetroRain

Posted on 10-26-13 02:38 AM (rev. 3 of 10-26-13 02:41 AM)

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How do you do greater-than or less-than with 6502? I want to check a range of values. If the value is #$00 - #$11, it will jump to $8FC7, if it is #$12 to #$13, it will jump to $8FCD, and if it is #$14 - #$19, it will jump to $8FD3. Granted, I could simpy do a bunch of CMPs, but I don't want to waste the space. Is there a better way to do it? Thanks.


$8FB9:A5 1B     LDA $1B = #$00
$8FBB:C9 00     CMP #$00
$8FBD:F0 08     BEQ $8FC7
$8FBF:C9 12     CMP #$12
$8FC1:F0 0A     BEQ $8FCD
$8FC3:C9 14     CMP #$14
$8FC5:F0 0C     BEQ $8FD3

$8FC7:A9 26     LDA #$26
$8FC9:8D 06 01  STA $0106 = #$02
$8FCC:60        RTS

$8FCD:A9 27     LDA #$27
$8FCF:8D 06 01  STA $0106 = #$02
$8FD2:60        RTS

$8FD3:A9 28     LDA #$28
$8FD5:8D 06 01  STA $0106 = #$02
$8FD8:60        RTS

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infidelity

Posted on 10-26-13 02:50 AM

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First, dont do a cmp of 00, waste of code. If you want to do a cmp of 00, you simply load the register, than follow it with a bne/beq, or whatever branch opcode it is that you want. When comparing multiple values, have your 00 cmp first, then you can follow with c901, f0##, c902, f0##.

never-obsolete

Posted on 10-26-13 02:53 AM

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-Here's an explanation on how cmp/cpx/cpy work and there's a table showing which branch instructions to use.

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

-An alternate to a long list of compares is a jump table, but that might take up more space.

RetroRain

Posted on 10-26-13 02:53 AM (rev. 2 of 10-26-13 02:58 AM)

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So, are you saying that there is no greater-than or less-than technique? I have no choice but to do a bunch of CMPs?

EDIT - Okay, never-obsolete posted at the same time I made my reply to infidelity's post.

So I can use either BMI, BPL, BCC, or the BCS?

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never-obsolete

Posted on 10-26-13 03:12 AM

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Yeah.

So to check if the accumulator is less then some value, n:


	cmp #n
	bcc _isLess
isGreaterEqualTo:
	; code if (A >= n)
	jmp endblock
isLess:
	; code if (A < n)
endblock:

The above is an if-then-else. This can be shortened if you don't need the else by branching on the opposite case:


	cmp #n
	bcs isGreaterEqualTo
isLess:
	; code if (A < n)
isGreaterEqualTo:

There are some tricky ones that require testing for equality first. BCS is one example, it branches when A >= n. So if you wanted to just branch on A > n, you'd first need a BEQ then the BCS. The link I posted covers this stuff.

RetroRain

Posted on 10-26-13 03:17 AM

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Ah, I see. Yeah, sounds a little bit trickier, but I see what you are saying. Alright, I appreciate your help. Thanks for the help both of you.

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Chaobomr

Posted on 10-26-13 06:58 AM

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The CMP statement is 6502's version of the infamous

if/then:else

statement?

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RetroRain

Posted on 10-26-13 08:02 PM (rev. 2 of 10-26-13 08:07 PM)

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Not by itself. The CMP just checks a value. You have to use other opcodes like BCC (Branch on Carry Clear) or BCS (Branch on Carry Set) like never-obsolete gave in the example above. The 6502 has special "status" flags that can be set/un-set, and depending on if the flag is set, you can branch to another spot in the code.

I have used BCC and BCS in my hack MM Nemesis, but I don't personally care for using them though. That's just me. I think because I don't have a lot of experience using them, and I don't like dealing with "flags". I like to simply stay with LDA, STA, CMP, JSR, and JMP opcodes.

Anyway, never-obsolete, after evaluating BCC/BCS, I have decided to use a table instead, because it actually suits my purposes later on down the road when it comes time to do some heavier hacking. It is a lot easier to read, and not all stages require long tables. It saves a lot of checking, and makes it easier to hack later on. But at least I know now how to do the greater-than/less-than stuff. It should come in handy for the future.

Thanks again for the help though.

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Chaobomr

Posted on 10-30-13 01:21 AM

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CMP BCC/BCS certainly sounds like those if/then statements that you see in many other languages

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