Beginning Amiga Assembly Programming

[pmc]

Hey there - gonna get back into coding cracktros and stuff? Would be cool

From your code, you could replace:

                    jsr                 _LVOOpenLibrary(a6)
                    tst.l               d0
                    beq.b               .no_lib
                    move.l              d0,a6

with:

                    jsr                 _LVOOpenLibrary(a6)
                    move.l              d0,a6
                    beq.b               .no_lib

but I'd say your 15 years old memory is working pretty good

[StingRay]

Originally Posted by pmc

                    jsr                 _LVOOpenLibrary(a6)
                    move.l              d0,a6
                    beq.b               .no_lib

but I'd say your 15 years old memory is working pretty good

This is buggy code because move dx,ax does not set any flags! The tst.l is required in his code.

[pmc]

LOL. That told me then!

[StingRay]

It's quite a common mistake actually, yours truly is guilty of having written such code too.

[matthey]

Originally Posted by Lonewolf10

I think it is necessary to write stupid comments in the beginning, because if anything else it helps to build ones confidence in using the instruction. Atleast if you forgot what it does you have an easy lookup.
I do think it's definately a good idea to write a "stupid comment" after instructions like move.l 4(a0,d0.w),d1 which are quite confusing for beginners - or atleast it was for me.

If the new style formatting was used, it would be a little easier to explain move.l (4,a0,d0.w),d1 for beginners. Think of a '+' everywhere there is a ',' gives move.l (4+a0+d0.w),d1. I used to use the old style formatting but gave it up because the new style is better (easier to read) and any decent assembler accepts the new style even for the 68000. There is no reason to teach new programmers old stubborn habits. At least you are using small caps in code for the instructions which is easier to read .

[Lonewolf10]

Originally Posted by matthey

If the new style formatting was used, it would be a little easier to explain move.l (4,a0,d0.w),d1 for beginners. Think of a '+' everywhere there is a ',' gives move.l (4+a0+d0.w),d1. I used to use the old style formatting but gave it up because the new style is better (easier to read) and any decent assembler accepts the new style even for the 68000. There is no reason to teach new programmers old stubborn habits.

I wasn't aware there was an old vs. new style for that. However, I am actually used to the old style so will probably stick with that

Originally Posted by matthey

At least you are using small caps in code for the instructions which is easier to read .

I don't understand why anyone would write (type) the instructions in capitals. The only thing I type all in capitals is the names of the registers, with some of the address labels and variables occasionally having capital letters (or underscores) to improve readability (e.g. yscreenposition vs. yScreenPosition).

Originally Posted by StingRay

This is buggy code because move dx,ax does not set any flags! The tst.l is required in his code.

I may have overlooked this in some of the manuals, but is there a list of which instructions set/clear the flags (other than obvious ones such as mulu/muls (for overflow) and divu/divs (for underflow))?

[matthey]

Originally Posted by Lonewolf10

I may have overlooked this in some of the manuals, but is there a list of which instructions set/clear the flags (other than obvious ones such as mulu/muls (for overflow) and divu/divs (for underflow))?

The M68000PRM shows how the condition codes are set for each instruction:

http://www.freescale.com/files/archi.../M68000PRM.pdf

Any OPA instruction (An destination), except CMPA, does not affect the CCR. Additionally, all OPA instructions with a word size immediate sign extend the data to a longword before the operation. Note that TST.W An does only test the lower word of An. Vasm (fixed in latest version) and PhxAss have an optimization bug that does CMPA.W #0,An -> TST.W An for 68020+ (TST An not allowed on 68000). This should be CMPA.W #0,An -> TST.L An. The bug went unreported for many years.

[pintcat]

Originally Posted by StingRay

This is buggy code because move dx,ax does not set any flags! The tst.l is required in his code.

Ugh... I've seen this often in other source codes which I was intended to learn from. I had no clue that this is actually pointless.
Another (nooby) question which might be a bit off-topic here: I know how to check a library version by calling the exec function OpenLib(). But what if I want to check the version of the exec.library itself? Do I have to use OpenLib() here too or are there other ways?
(OpenLib wouldn't work on Kickstarts below 1.1 and OldOpenLib doesn't check the version, that's why I'm asking)

[Leffmann]

The library version is stored in the library base, so in the case of exec it would be SysBase->LibNode.lib_Version, it's an unsigned 16-bit word at offset 20 from the library base.

[pintcat]

Yay - that sounds easy even to me! Thanx for the hint Leffmann. Now I have to finish some code...

[redblade]

Originally Posted by StingRay

This is buggy code because move dx,ax does not set any flags! The tst.l is required in his code.

Glad I read this thread, I make that mistake :/

[AlfaRomeo]

Someone could explain me the difference between brs and jsr?
Thanks in advance

[StingRay]

bsr and jsr are basically the same commands, jsr just allows a larger branch distance than bsr. If you need to call a routine in a different section you need to use jsr for example. I'd just always use bsr, your assembler will give you an error message if the distance is too large, then you can change the bsr to jsr.