Re: QUERY : ARM mode inline instructions in C Thumb mode file?

hello:

one small question regarding use of ARM inline assembly code in a
C file that has been compiled for Thumb mode.

is it possible to use ARM assembly code from within a C file that
has been compiled for Thumb and Thumb interworking?

how this is done is described on this page:
http://www.devrs.com/gba/files/asmc.txt

i have a C file that has been compiled for Thumb mode. in it, i am
using ARM inline assembly code. apparently, GCC issues no error
message but forcibly converts the ARM code into Thumb code.

i think that GCC requires an entire file to be in either Thumb mode
or ARM mode, but i am not sure. is that true?

the C code that is compiled into Thumb is:

void function_f( void )
{
asm volatile
(
".align \n\t"
".arm \n\t"
"mrs r0, cpsr \n\t"
"msr cpsr_c, r0 \n\t"
"pop { r0 }"
);
}

the C code is compiled with:
arm-elf-gcc -mlittle-endian -mcpu=arm7tdmi -march=armv4t \
-mthumb-interwork -mthumb -mno-tpcs-frame .....

here is the generated code:
00008194 <function_f>:
8194: b580 push {r7, lr}
8196: af02 add r7, sp, #8
8198: 0000 lsls r0, r0, #0
819a: e10f b.n 83bc <wrap+0x10>
819c: f000 e121 blx 4083e0 <_stack+0x3883e0>
81a0: 0001 lsls r1, r0, #0
81a2: e8bd 46bd ldmia.w sp!, {r0, r2, r3, r4, r5, r7, r9,
sl, lr}
81a6: b082 sub sp, #8
81a8: bc80 pop {r7}
81aa: bc01 pop {r0}
81ac: 4700 bx r0


in the inline assembly code, if i do not use ".arm", i get compiler
errors.

is it possible to use ARM assembly code from within a C file that
has been compiled for Thumb and Thumb interworking?

Aaron

Makefile
--------

CC = arm-elf-gcc
TARGET_ARCH = -mlittle-endian -mcpu=arm7tdmi -march=armv4t
TARGET_ARCH += -mthumb -mno-tpcs-frame -mthumb-interwork

all: test
arm-elf-objdump -S -D test > test.lst

clean:
rm -f test test.o function.o


C CODE
------

void function_f( void )
{
asm volatile
(
".align \n\t"
".arm \n\t"
"mrs r0, cpsr \n\t"
"msr cpsr_c, r0 \n\t"
"pop { r0 }"
);
}

int main( void )
{
function_f();
return 0;
}

Try this:

void function_f( void )
{
asm volatile
(
".code 32 \n\t"
"mrs r0, cpsr \n\t"
"msr cpsr_c, r0 \n\t"
".code 16 \n\t"
"pop { r0 }"
);
}

It generates:

function_f:
.code 32
mrs r0, cpsr
msr cpsr_c, r0
.code 16
pop { r0 }
bx lr

You need to return to thumb mode after inserting the arm code or the
assembles will get confused.
BTW, that "pop r0" will crash the program.

regards,
Giovanni Di Sirio
---
ChibiOS/RT http://chibios.sourceforge.net

.

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