Please help with ASM

[cometkeeper]

Hallo, I'm trying to understand some differences between intel and at&t asm.

This intel code works ok. It stores a char into the video memory.

start:
mov ax,0xb800
mov ds,ax
mov si,0x0
mov byte [ds:si],0x22
inc si
mov byte [ds:si],0x07
start1:
jmp start1

I tried to translate it into the at&t asm but I cannot understand where the problem is. Can anyone translate it for me????

OTHER QUESTION if I may:

I'm using djgpp.

I have problem when I try to use asm in my .c file. I have to separate each line with the " otherwise I get an error.
When I use this way and I need to specify some in/out/reg. I get another error: "asm template is not a string constant"

__asm__("movw %0,%%ax" : : "=g" (i) );

this generate that error.... Does anyone know where is the problem?

Thank you and sorry if I kept it long...

[frank]

hallo,

Are you dutch/belgium?

I have problem when I try to use asm in my .c file. I have to separate each line with the " otherwise I get an error.

Use the ; to seprate, so like this

Code: Select all

asm volatile("mov $3,%ax";
                          "mov $4,%bx");

Btw, you can also just use a pointer into the memory
( int * mem ) for writing to the video memory.
I don't know if this is faster in anyway (it might get optimized by the compiler)

[Schol-R-LEA]

waits for ears to stop ringing from Cometkeeper's shouting in the Subject line

Hmmn, this probably should be in the General Programming forum... Anyway, here's a quick-and-dirty translation of you code into AT&T syntax. This has not been tested, but it should be what you want:

Code: Select all

start:
        movw 0xb800, %ax
        movw %ax, %ds
        movw 0x00, %si
        movb 0x22, %ds:(%si)
        incw si
        movb 0x07, %ds:(%si)
start1:
        jmp start1

Keep in mind that if you are using this for 16-bit real mode programming, you need to add the directive

Code: Select all

.code16

at the beginning of the code.

For more details on AT&T syntax, see reply #4 in this thread. HTH. C&CW.

[Therx]

Very OT

C&CW.

Huh. What's that an abbreviation for?

Pete

[Schol-R-LEA]

"Comments and Corrections Welcome". That is to say, if I made a mistake, or if you have something productive to add, feel free to add speak you mind about it. Similarly, "CCW" (without the ampersand) means "Constructive Criticism Welcome".

[Pype.Clicker]

hmm ... i haven't looked deeply at it, but using "ds <= b800" in a DjGpp environment makes me feel the urge to remind you that DjGPP is a 32 bits compiler, and 32bits mode is unlikely to allow a *selector* that has value b800 (typical from real mode addresses)

[cometkeeper]

Ok, I'll try to explain the problems:

this is may .c

----test.c----
int main(void)
{
   int i;

   __asm__("movw $0x1, %ax\n
       movw $0x2, %bx");
   
   return 0;
}

This generate the error:
test.c:5:17: missing terminating " character
test.c: In function `main':
test.c:6: error: parse error before "movw"
test.c:6:32: missing terminating " character

I compiled with
gcc test.c (maybe I miss some compiler switch???)

To solve this problem I have to do this

__asm__("movw $0x1, %ax\n movw $0x2, %bx");

This doesn't get any error, but when you ave long asm code is

impossible to write every instruction in a row.

Second problem:

__asm__("movw %%ax,%0": : "=g" (i));

generates:
test.c: In function `main':
test.c:5: error: input operand constraint contains `='

I knew that the output operand was the second
__asm__("asm-stat":input:output:register);
Isn't it????

If I separate the row I don't get any error

__asm__("movw $0x1, %ax",
    movw $0x2, %bx");

but

__asm__("movw %%ax, %0",
    movw %%ax, %%bx"
   :
   : "=g" (i)
   );

gets:
test.c: In function `main':
test.c:9: error: asm template is not a string constant


And... In reply to the translated code I get this error at:

movb $0x22, %ds:(%si)

c:/djgpp/tmp/cchS4U2H.s: Assembler messages:
c:/djgpp/tmp/cchS4U2H.s:12: Error: `%ds:(%si)' is not a valid 32 bit base/index
expression


Thank you....

[Pype.Clicker]

Ok, I'll try to explain the problems:

this is may .c

----test.c----
int main(void)
{
int i;

__asm__("movw $0x1, %ax\n
movw $0x2, %bx");

return 0;
}

This generate the error:
test.c:5:17: missing terminating " character
test.c: In function `main':
test.c:6: error: parse error before "movw"
test.c:6:32: missing terminating " character

I compiled with
gcc test.c (maybe I miss some compiler switch???)

Hum. Seems like a little browsing of 'info gcc' may help you

The problem here is that multi-line strings are not really standard in C. Some compilers/version support them, other don't. However, i think ansi-C forces the compiler to concatenate strings separate only by "blank" symbols, thus you could write

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asm ("push es\n"
       "pop ds");

and it should work fine ...

Second problem:

__asm__("movw %%ax,%0": : "=g" (i));

generates:
test.c: In function `main':
test.c:5: error: input operand constraint contains `='

I knew that the output operand was the second
__asm__("asm-stat":input:output:register);
Isn't it????

I wouldn't bet on this ... it's more likely to be
asm ("asm-stat":output:input:scratched);

And... In reply to the translated code I get this error at:

movb $0x22, %ds:(%si)

c:/djgpp/tmp/cchS4U2H.s: Assembler messages:
c:/djgpp/tmp/cchS4U2H.s:12: Error: `%ds:(%si)' is not a valid 32 bit base/index expression

you needn't to explicitly require '%ds:' : it's the default segment register... however, (%si) is -- as the assembler tells you -- a 16 bits addressing mode. (%esi) would be more likely to be accepted. I think if you want to use 16 bits addressing with AS, you have to put the A16 prefix override for each instruction, though i never tried it myself ...