poke function ....

Art Berggreen art@acc.com
Fri Sep 18 15:42:00 GMT 1998


>
>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                                     */
>
>	mov ax,[base_addr]
>	mov es,ax
>	mov al,[bp+0x0C]
>	mov si,[bp+0x0A]
>	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??
>Is there anybody M68K programmer know any C function to replace
>pokeb, peekb function in M68K??

I don't know the 80x86 machine architecture or 80x86 C calling
interface, but I'll make a few guesses.

It looks like pokeb() takes a variable holding a segment number,
an offset into that segment, and data to write into memory ("poke").

>	mov ax,[base_addr]
>	mov es,ax

Loads the content of the "base_address" variable into register es.

>	mov al,[bp+0x0C]

Loads the value of "data" which is being passed on the stack at a stack
offset of 0x0c into register al.

>	mov si,[bp+0x0A]

Loads the value of "offset" which is being passed on the stack at a stack
offset of 0x0a into register si.

>	mov es:[si],al

Stores the data in register al into memory at the location pointed to
by the segment in register es and the offset in register si.

Since the m68k is not a segmented memory machine, the pokeb() function
wouldn't need a base and offset, but in the interest of following the
same model, an m68k routine could look like:

/* pokeb(char *base_addr, int offset, char data) */
	.global pokeb
pokeb:
	move.l	4(%sp),%a0	/* get base_addr */
	add.l	8(%sp),%a0	/* add offset */
	move.b	15(%sp),(%a0)	/* move data byte */
	rts

Art



More information about the crossgcc mailing list