changing data in function

[ghjjkl]

Hello again, i have some new novice's problem: how to pass address of some data to change it in function? I tried something like "invoke func,offset _address" and then, for example, in function: "mov ax, word ptr @data:[_address_which's_argument]", but it doesnt work, and ax contains yet another value i can't recognize   :(

[nidud]

deleted

[dedndave]

i have to admit that i have never tried INVOKE in 16-bit code, but there's no reason it shouldn't work
that's because, in DOS days, i used MASM v 5.1 which did not support INVOKE

INVOKE'd functions require a PROTOtype, generally placed early in the file
(well - they require one if the INVOKE is to appear before the PROC)
the prototype tells the assembler how many arguments to expect, and their sizes
it may also tell the assembler which type of calling convention to use and the distance (NEAR or FAR)
for 32-bit programs, everything is NEAR, and the default is StdCall (defined in the .MODEL directive)
i am not sure how that works in 16-bit code
you'll have to consult the MASM manual for details

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MyFunc  PROTO  <dist> <type> :WORD,:WORD,:WORD
<dist> would be replaced with NEAR or FAR
<type> would be C, Pascal, Basic, StdCall, SysCall, etc
the default depends on the .MODEL and OPTION's (read the manual)
let's assume you use NEAR and StdCall.....

then, in the code section...

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        .CODE

        INVOKE  MyFunc,arg1,arg2,arg3

and, the function

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MyFunc PROC  NEAR StdCall _arg1:WORD,_arg2:WORD,_arg3:WORD

        mov     ax,_arg1
        mov     cx,_arg2
        mov     dx,_arg3
;
;
        ret

MyFunc ENDP

because you use NEAR and StdCall, the actual code generated might look like this

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        push    arg3
        push    arg2
        push    arg1
        call    MyFunc

and....

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MyFunc:
        push    bp
        mov     bp,sp

        mov     ax,[bp+4]
        mov     cx,[bp+6]
        mov     dx,[bp+8]
;
;
        leave
        ret     6

the LEAVE instruction is essentially the same as

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        mov     sp,bp
        pop     bp

RET 6 returns and POP's 6 bytes off the stack (the passed arguments)

the behaviour is different for other <dist> and <type> parameters

[ghjjkl]

um... i understand now, i didn't consider these problems to be the same... so sorry for this topic then and thanks for attention  :(

[ghjjkl]

and what if i'll want to pass STRUC's address in function? Should i consider its members as [bx+number_of_bytes_before_some_member]?

[nidud]

deleted

[ghjjkl]

wow, that's cool  :t
what does "assume " mean with registers like bx? I used "assume" with registers like ds,cs,ss,es so far

[nidud]

deleted

[dedndave]

normally, we would pass the address of the entire structure

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        INVOKE  MyFunc,offset S

then, in the routine, address the individual member(s)
you can use ASSUME, as nidud mentioned - but you don't have to

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MyFunc PROC pStruct:WORD

        mov     bx,pStruct
        mov     al,[bx].S1.m1
        mov     cx,[bx].S1.m2

you could also pass the address of an individual member

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        INVOKE  MyFunc,offset S.m2

[ghjjkl]

But what about arrays of STRUC's? If i want to change some members, using cycle,i could try:

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some struc
a_ dw ?
b_ dw ?
some ends

.data

_some some 10 dup (<0,0>)

.code

mov ax,@data
mov ds,ax

xor bx,bx
.while bx<10
mov _some[bx].a_,5
inc bx
.endw

but it doesnt work.
Well, should i try this?

Code Select Expand


some struc
a_ dw ?
b_ dw ?
some ends

.data

_some some 10 dup (<0,0>)

.code

mov ax,@data
mov ds,ax

xor bx,bx
mov ax,10
cwd
mov dx,sizeof some
mul dx
.while bx<ax
mov _some[bx].a_,5
add bx,sizeof some
.endw


[dedndave]

see if this works...

(see next post)

[dedndave]

nix that one
try this

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some struc
a_ dw ?
b_ dw ?
some ends

.data

_some some 10 dup (<0,0>)

.code

mov ax,@data
mov ds,ax

xor bx,bx
.while bx<(10*sizeof some)
mov [bx]._some.a_,5
add bx,sizeof some
.endw

[ghjjkl]

no, sorry, it doesnt work.... should there be "mov [bx].some.a_,5" instead "mov [bx]._some.a_,5"?

[dedndave]

maybe

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    mov     _some[bx].some.a_,5

_some gets you the address
some gets you the structure offsets

[ghjjkl]

wow, it works! Thanks, finally i got it