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In a message dated 18-Sep-98 18:46:31
Dony <dony@willowglen.com.sg> wrote:
>Hii all,
>Right now, I'm trying to porting 80x86 code to M68K.
>In Intel, there is a function: pokeb(base_addr,offset,data)
>and using Borland C Compiler will translated like this :
> /* Assembly Routine to do a pokeb */
> /* bp points at stack */
> /* stack + 0xc = data */
> /* stack + 0xa = register offset from base addr. */
> /* es - set to be segment value (CS) */
> /* si - set to be register offset */
> /* al - data */
1> mov ax,[base_addr]
2> mov es,ax
3> mov al,[bp+0x0C]
4> mov si,[bp+0x0A]
5> mov es:[si],al
>I'm a high level programmer, and I don't really understand
>where this 0x0c and 0x0a come from, and how could in 0x0c
>is data, and in 0x0a is register offset??
>Can somebody explain step by step what the assembler
>trying to do??
1) Read "base_addr" into ax-register (16 bits),
2) Copies ax to es-register (probably due to cpu not beeing capable to load
it there directly from ram).
3) Reads al-register (8 bits) from the stack.
The offset of 0x0c (12 bytes) must be compiler/cpu-dependent - I don't
remember what info an intel cpu puts on the stack for return addresses.
4) Then does the same with si-register (16 bits)
5) Stores al-register contents at 20-bit adress given by es:[si]
16 bits of es and si *overlapped* and added such that they form 20 bits:
aaaa <- es contents, 4 bits per char
+ bbbb <- si contents, 4 bits per char
------
zzzzz <- 20-bit sum, to be used as physical address.
>Is there anybody M68K programmer know any C function to replace
>pokeb, peekb function in M68K??
pokeb == use a pointer and write char into RAM.
peekb == use a pointer and read char from RAM.
/Hannu E K Nevalainen http://www.it.kth.se/~henk/ _ // 1985+
mr_Henk @ #AmigaSWE/IRCNet \X/Amiga user
Personal rc5-stats: 8,805 2^28-key blks, i.e. 2,363,574+ Mkeys, M68K Amiga.
--
TKGTTA: How to call a routine without knowing where it is