To: djgpp AT delorie DOT com
Subject: ASM in DJGPP V2
Date: Wed, 18 Dec 1996 21:01:33 +0000
Message-ID: <2615.850942893@cs.ucl.ac.uk>
From: Chris AUSTIN <C DOT Austin AT cs DOT ucl DOT ac DOT uk>

I am new to using DJGPP but have been using Borland C++ 3.1 for a couple of
years.  I am currently trying to get a program running under DJGPP V2 that was
working in BC++, but am having problems with a bit of assembler for copying a 16x16 picture
to a virtual screen (s) which has width (s.width) and is stored as an array of unsigned
char (s.data).  The following section of code is the C++ version:-

                for (int row=0; row<16; row++)
                    memcpy(&s.data[(y+row)*s.width+x],&data[row*16],16);

which works in both, and the following is assembler which worked in BC++:-

		unsigned char* b=s.data+y*s.width+x;
		const unsigned char* p=data;
		int scrw=s.width;

		asm{
			push ds;

			mov ax,16;
			mov dx,16;
			mov bx,scrw;
			sub bx,ax;

			les di,b;
			lds si,p;

			cld;
			shr ax,1;
		}
		rowloop:
		asm{
			mov cx,ax;
			rep movsw;
			add di,bx;
			dec dx;
			jnz rowloop;
			pop ds;
		}

I have tried to convert this to GNU asm and came up with the following, but
this doesn't work.

What is wrong with it?

		register unsigned long b asm ("%edi") = s.data+y*s.width+x;
		register unsigned short offset asm ("%ebx") = s.width-16;
                register unsigned long d asm ("%esi") = &data[0];

		asm volatile (" cld                \n"
                              " push %%es          \n"
                              " movw %w0,%%es      \n"
                              "rowloop:            \n"
                              " movl %%eax,%%ecx   \n"
                              " rep; movsb         \n"
                              " addl %%ebx,%%edi   \n"
                              " decl %%edx         \n"
                              " jnz rowloop        \n"
                              " pop %%es             "
                              :
                              : "rm" (_my_ds()), "a" (16), "d" (16),
                                "b" (offset), "S" (d), "D" (b)
                              : "%eax", "%ebx", "%ecx", "%edx",
                                "%esi", "%edi", "cc");

I have read through the FAQ and the info pages but can't find that much
information on asm in DJGPP programs.  The bit of asm that I have written is based very
much on an example program in EXE magazine, so I don't fully understand it.  So if
someone could explain what the "rm" is in the input list and the "cc" means, I would
be grateful.  Thanks for any help in finding what is wrong with the code,

Chris Austin