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I'm trying to access the BIOS divide utility (SWI 6) and I'm having trouble inlining the ASM calls. Here's the source code:

Code:

void BIOSDIV(
signed long numerator,
signed long denominator,
signed long *div,
signed long *mod,
signed long *abs
){

asm volatile(
   "
@ SUBSTITUTION LIST
@ 0: %0
@ 1: %1
@ 2: %2
@ 3: %3
@ 4: %4

ldr    r0,=%3 @ Load the value of %3(numerator) into r0
ldr    r1,=%4 @ Load the value of %4(denominator) into r1
swi    6    @ Call BIOS SWI 6 (DIV)
str    r0,[%0] @ Store value of r0 (numerator DIV denominator) into &div
str    r1,[%1] @ Store value of r1 (numerator MOD denominator) into &mod
str    r2,[%2] @ Store value of r2 (abs of numerator DIV denominator) into &abs
   "
   : "=r"(div), "=r"(mod),"=r"(abs) // OUTPUT LIST
   : "r"(numerator), "r"(denominator) // INPUT LIST
   : "r0","r1","r2","r3","r4","r5"    // CLOBBER LIST
);
}

What I get from the compiler is:

Code:

test.o: In function `BIOSDIV(long, long, long*, long*, long*)':
test.o(.text+0x88): undefined reference to `lr'
test.o(.text+0x8c): undefined reference to `ip'

A look at the asm file that is generated by the compiler shows:

Code:

_Z7BIOSDIVllPlS_S_:
@ Function supports interworking.
@ args = 4, pretend = 0, frame = 0
@ frame_needed = 0, current_function_anonymous_args = 0
stmfd sp!, {r4, r5, r6, lr}
mov lr, r0
mov ip, r1

@ SUBSTITUTION LIST
@ 0: lr
@ 1: r6
@ 2: ip
@ 3: lr
@ 4: ip

ldr    r0,=lr @ Load the value of lr(numerator) into r0
ldr    r1,=ip @ Load the value of ip(denominator) into r1
swi    6    @ Call BIOS SWI 6 (DIV)
str    r0,[lr] @ Store value of r0 (numerator DIV denominator) into &div
str    r1,[r6] @ Store value of r1 (numerator MOD denominator) into &mod
str    r2,[ip] @ Store value of r2 (abs of numerator DIV denominator) into &abs

ldmfd sp!, {r4, r5, r6, lr}
bx lr

Why is this in C++ inline ASM:

Code:

@ SUBSTITUTION LIST
@ 0: %0
@ 1: %1
@ 2: %2
@ 3: %3
@ 4: %4

Turning into this in generated ASM:

Code:

@ SUBSTITUTION LIST
@ 0: lr
@ 1: r6
@ 2: ip
@ 3: lr
@ 4: ip

Does anyone have any idea why the substitions aren't working? I'm totally new at inlining ASM so any help would be appreciated.

Thanks!

Well, appearently it's using all sorts of registers for the inputs, but I always thought it used the stack instead.
Well I use this, and I know it works, so try it instead:

void divfull(s32 numerator, s32 denomenator,
s32 *quotient, s32 *remainder, s32 *abs)
{
asm volatile
("
mov r0, %3
mov r1, %4
swi 0x60000
ldr r2, %0
str r0, [r2]
ldr r2, %1
str r1, [r2]
ldr r2, %2
str r3, [r2]
"
: "=m" (quotient), "=m" (remainder), "=m" (abs)
: "r" (numerator), "r" (denomenator)
: "r0", "r1", "r2", "r3"
);
}

Ah, there was the problem, you were putting "=r"(output) instead of "=m"(output), so it was just picking registers to put those in. No idea why it would pick so many important ones, especially ip, but oh well^_^

DekuTree64 wrote:

Ah, there was the problem, you were putting "=r"(output) instead of "=m"(output), so it was just picking registers to put those in. No idea why it would pick so many important ones, especially ip, but oh well^_^

I don't seem to grasp the difference... the documentation says that "r" simply means a generic register but that "m" means a memory address.

Quote:

r
A register operand is allowed provided that it is in a general register.

m
A memory operand is allowed, with any kind of address that the machine supports in general.

Isn't a general register able to handle a memory address? I'm not sure I understand the black magic that's going on behind the scenes...
_________________
"Beer is proof that God loves us and wants us to be happy."
-- Benjamin Franklin

Also another question: is the BIOS divide utility more efficient than using the "/" and "%" operators in C?
_________________
"Beer is proof that God loves us and wants us to be happy."
-- Benjamin Franklin

The answer to that is a resounding and very big YES ;)

Another question: is it possible with C++ to make an operator use a different function, somewhat like operator overloading, but with primitive types? Essentially, is it possible to replace the "/" and "%" operators to call a function that retreives their respective values after a BIOS divide instead? Like, make the "/" operator point to and call a function that does a BIOS divide and return the quotient and "%" do a BIOS divide and return the modulus?
_________________
"Beer is proof that God loves us and wants us to be happy."
-- Benjamin Franklin

animension wrote:

Another question: is it possible with C++ to make an operator use a different function, somewhat like operator overloading, but with primitive types?

C++ does not support replacing the primitive types' operator actions. You'll need to make a class.
_________________
-- Where is he?
-- Who?
-- You know, the human.
-- I think he moved to Tilwick.

DekuTree64 wrote:

Well, appearently it's using all sorts of registers for the inputs, but I always thought it used the stack instead.
Well I use this, and I know it works, so try it instead:

void divfull(s32 numerator, s32 denomenator,
s32 *quotient, s32 *remainder, s32 *abs)
{
asm volatile
("
mov r0, %3
mov r1, %4
swi 0x60000
ldr r2, %0
str r0, [r2]
ldr r2, %1
str r1, [r2]
ldr r2, %2
str r3, [r2]
"
: "=m" (quotient), "=m" (remainder), "=m" (abs)
: "r" (numerator), "r" (denomenator)
: "r0", "r1", "r2", "r3"
);
}

I cut and pasted this and I get the following compiler errors:

Code:

---------- Compile ROM ----------
test.cpp: In function `void divfull(int, int, s32*, s32*, s32*)':
test.cpp:34: output number 1 not directly addressable
test.cpp:34: output number 2 not directly addressable
test.cpp:50: confused by earlier errors, bailing out
make: *** [test.o] Error 1
Output completed (0 sec consumed) - Normal Termination

_________________
"Beer is proof that God loves us and wants us to be happy."
-- Benjamin Franklin

Hmm, well I copy/pasted it straight from my game, so I don't know why it wouldn't work. Well, try using your code, but just replacing the "=r" with "=m" and see if that fixes it.
And about your question on the difference between r and m, at least as far as I know, you need to use m when you'll be storing something back to memory, and r when you just want to put it in a register and leave it there. I'm not too clear on inline ASM myself though, I pretty much copy/pasted and guessed my way into getting it working. One of these days I'll get around to making a pure ASM div function instead though, so you might want to just do that to begin with too.

This worked for me, but it's not the most efficient:

Code:

/* bios_divmod() ***********************
Convert n/d to a mixed fraction q+r/d
Returns Q.
*/
int bios_divmod (int num, int den, int *rem)
{
int quo;

asm volatile (
" mov r0,%2 \n"
" mov r1,%3 \n"
" swi 6 \n" /* in ARM, s/6/0x60000/ */
" mov %0,r0 \n"
" ldr r2,%1 \n"
" str r1,[r2]\n"
: "=r" (quo), "=m" (rem) // Outputs
: "r" (num), "r" (den) // Inputs
: "r0","r1","r2","r3" // clobber
);
return quo;
}

Normally, I use this external asm subroutine, which even handles divide-by-zero in a rational manner (important for some projection code):

Code:

@ int dv(int num, int den)
@ Divide two signed integers.

.THUMB
.THUMB_FUNC
.ALIGN
.GLOBL dv

dv:
cmp r1, #0
beq 0f
swi 6
bx lr
0:
ldr r0, =0x7fffffff
bx lr

[/code]
_________________
-- Where is he?
-- Who?
-- You know, the human.
-- I think he moved to Tilwick.

gbadev.org forum archive

ASM > Help with some inline ASM

#1275 - animension - Thu Jan 16, 2003 4:00 am

#1305 - DekuTree64 - Thu Jan 16, 2003 5:22 pm

#1306 - DekuTree64 - Thu Jan 16, 2003 5:26 pm

#1326 - animension - Thu Jan 16, 2003 7:48 pm

#1343 - animension - Thu Jan 16, 2003 9:23 pm

#1344 - Splam - Thu Jan 16, 2003 9:28 pm

#1346 - animension - Thu Jan 16, 2003 10:17 pm

#1355 - tepples - Thu Jan 16, 2003 11:56 pm

#1358 - animension - Fri Jan 17, 2003 12:06 am

#1367 - DekuTree64 - Fri Jan 17, 2003 5:30 am

#1399 - tepples - Sat Jan 18, 2003 5:23 am