C言語→アセンブリ

以下のソースをThumb(16bit)、ARM(32bit)へコンパイルして機械語の長さ(ビット数)を確認する

test.c

int add(int a, int b) {
    return a + b;
}

C言語→Thumb

機械語を確認すると全て16bit長になっている。

testuser@CasaOS:~/test$ arm-none-eabi-gcc -mthumb -mcpu=cortex-m3 -O0 -c test.c -o test.o
testuser@CasaOS:~/test$ arm-none-eabi-objdump -d test.o

test.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <add>:
   0:   b480            push    {r7}
   2:   b083            sub     sp, #12
   4:   af00            add     r7, sp, #0
   6:   6078            str     r0, [r7, #4]
   8:   6039            str     r1, [r7, #0]
   a:   687a            ldr     r2, [r7, #4]
   c:   683b            ldr     r3, [r7, #0]
   e:   4413            add     r3, r2
  10:   4618            mov     r0, r3
  12:   370c            adds    r7, #12
  14:   46bd            mov     sp, r7
  16:   bc80            pop     {r7}
  18:   4770            bx      lr

C言語→ARM

機械語を確認すると全て32bit長になっている。

testuser@CasaOS:~/test$ arm-none-eabi-gcc -marm -mcpu=cortex-a9 -O0 -c test.c -o test_arm.o
testuser@CasaOS:~/test$ arm-none-eabi-objdump -d test_arm.o

test_arm.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <add>:
   0:   e52db004        push    {fp}            @ (str fp, [sp, #-4]!)
   4:   e28db000        add     fp, sp, #0
   8:   e24dd00c        sub     sp, sp, #12
   c:   e50b0008        str     r0, [fp, #-8]
  10:   e50b100c        str     r1, [fp, #-12]
  14:   e51b2008        ldr     r2, [fp, #-8]
  18:   e51b300c        ldr     r3, [fp, #-12]
  1c:   e0823003        add     r3, r2, r3
  20:   e1a00003        mov     r0, r3
  24:   e28bd000        add     sp, fp, #0
  28:   e49db004        pop     {fp}            @ (ldr fp, [sp], #4)
  2c:   e12fff1e        bx      lr

直接アセンブリで書く

Thumb(16bit)、ARM(32bit)それぞれ実行できるかを確認する。

Thumb

ThumbでADDS r0, r0, r1は使えない。 adds r0, #2`のように片方は即値

teat.s

.global _start
.thumb
.thumb_func
.syntax unified
_start:
    MOVS r0, #1
    ADDS r0, #2
    MOVS r7, #1
    SVC 0

testuser@CasaOS:~/atest$ as -o test.o test.s -mcpu=cortex-a5 -mthumb
testuser@CasaOS:~/atest$ ld -o test test.o -e _start
testuser@CasaOS:~/atest$ objdump -d test

test:     file format elf32-littlearm


Disassembly of section .text:

00010054 <_start>:
   10054:       2001            movs    r0, #1
   10056:       3002            adds    r0, #2
   10058:       2701            movs    r7, #1
   1005a:       df00            svc     0
testuser@CasaOS:~/atest$ gdb ./test
GNU gdb (Debian 13.1-3) 13.1
Copyright (C) 2023 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "arm-linux-gnueabihf".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./test...
(No debugging symbols found in ./test)
(gdb) break *0x1005a
Breakpoint 1 at 0x1005a
(gdb) run
Starting program: /home/testuser/atest/test

Breakpoint 1, 0x0001005a in _start ()
(gdb) info registers r0
r0             0x3                 3

ARM(32bit)

teat.s

.global _start

.arm
_start:
    MOV r0, #1
    MOV r1, #2
    ADD r0, r0, r1
    MOV r7, #1       @ syscall: exit
    SVC 0

testuser@CasaOS:~/atest$ as -o test.o test.s
testuser@CasaOS:~/atest$ ld -o test test.o -e _start
testuser@CasaOS:~/atest$ objdump -d test

test:     file format elf32-littlearm


Disassembly of section .text:

00010054 <_start>:
   10054:       e3a00001        mov     r0, #1
   10058:       e3a01002        mov     r1, #2
   1005c:       e0800001        add     r0, r0, r1
   10060:       e3a07001        mov     r7, #1
   10064:       ef000000        svc     0x00000000
testuser@CasaOS:~/atest$ gdb test
GNU gdb (Debian 13.1-3) 13.1
Copyright (C) 2023 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "arm-linux-gnueabihf".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from test...
(No debugging symbols found in test)
(gdb) break *0x10064
Breakpoint 1 at 0x10064
(gdb) run
Starting program: /home/testuser/atest/test

Breakpoint 1, 0x00010064 in _start ()
(gdb) info registers r0
r0             0x3                 3
(gdb)

動作環境（Linux / ARM 版）

OS: Armbian 23.08.0-trunk (Debian 12 Bookworm)
Kernel: Linux 6.1.38-meson (armv7l)
CPU: ARM Cortex-A5, 4 cores, Little Endian
Architecture: armv7l
GNU assembler: 2.40 (arm-linux-gnueabihf) using BFD version (GNU Binutils for Debian) 2.40
GNU ld: (GNU Binutils for Debian) 2.40
GDB: GNU gdb (Debian 13.1-3) 13.1
備考: Windows10からSSH接続して使用