QUERY : ARM inline code in Thumb file?

[Aaron P. D'Souza]

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"
    "__f_into_arm:\n\t"
    ".arm                  \n\t"
    "mrs     r0,     cpsr  \n\t"
    "msr     cpsr_c, r0    \n\t"
    "pop     { r0 }        \n"
    "__f_from_arm:\n\t"
    ".thumb"
  );
}

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

00008198 <__f_into_arm>:
    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}

000081a4 <__f_from_arm>:
    81a4:  b08246bd    strlth  r4, [r2], sp
    81a8:  bc01bc80    stclt   12, cr11, [r1], {128}
    81ac:  00004700    andeq   r4, r0, r0, lsl #14

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?

Makefile
--------

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

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

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


test.c
-------

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

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

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