Re: asm and nasm newbie

Zhang Huan wrote: 
hi all:
    want to get parameter from stack, i use "mov di, [sp+4]". while
compiling error occurs "invalid effective address". what's wrong


It's not a valid addressing mode. :)

16-bit addressing modes are *quite* lame, compared to 32-bit addressing modes - only bx, si, di, and bp are useable (and there are further restrictions on combining them - bx and bp are "base" registers, si and di are "index" registers - you can only use one of each. [bx + si] is valid, [si + di] is not).

32-bit instructions are much more flexible. You can use *almost* any combination of registers you want - except that esp can't be an index register - [eax + esp * 4] is not valid - [esp + eax * 4] is. You can add an offset to any of these.

There's a section on "effective address encoding" in the Friendly Nasm Manual, but I doubt if a beginner will understand it... I don't!

You can use 32-bit instructions in 16-bit code. Nasm *does* understand this, and will add the 67h address size override prefix automatically (as Wolfgang explains). Nasm will even figure out that [eax * 5] can legitimately be coded [eax + eax * 4], and will do so "behind your back". (I wouldn't advise you to actually *use* this.)

If you expect that your 16-bit code will actually run on a 16-bit CPU, use "push bp", "mov bp, sp", etc. as Wolfgang explains, If not (and who's got a 16-bit CPU these days???), Nasm will let you "mix and match" 16- and 32-bit instructions, and will insert the 66h and 67h override bytes as appropriate. There's a small "penalty" in size and speed for doing so, but it's often a "win" anyway.

32-bit coding is a lot easier than 16-bit, in many ways. Interfacing with existing 32-bit OSen is... different. In *my* opinion, it's "harder" (Linux easier than Windows), but some people claim that Windows programming is "just as easy" as dos...

For an introduction to 32-bit coding, see Dr. Paul Carter's tut:

http://www.drpaulcarter.com/pcasm

He uses the C library for the OS-specific stuff, so it'll work with Windows or Linux (and other Unixen?) or djgpp (... but it uses the C library for OS-specific stuff...).

Move on from there when you feel "ready", I guess...

Best,
Frank

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