The following typedef example defines the bit fields in an I-Format instruction for a MIPS processor.
typedef union {
struct {
signed int immed:16;
unsigned int rt:5;
unsigned int rs:5;
unsigned int opcode:6;
};
unsigned int w;
} I_format_t;
Examination of the disassembly listing from the MPLAB IDE shows that the ext (extract bitfield) instruction is used to reference individual fields in the instruction.
20: printf("rs: %d rt: %d\n", ia.rs, ia.rt);
lw v0,16(s8) vo <= ia.w
ext v1,v0,21,5 v1 <= extract_bitfield(v0,21,5) note 1
lw v0,16(s8) vo <= ia.w note 2
ext a2,v0,16,5 a2 <= extract_bitfield(v0,16,5)
lui v0,0x9d07 a0 <= format_string
addiu a0,v0,16184
addu a1,v1,zero a1 <= v1
jal 0x9d000554 call printf
nop
The ext (extract bitfield) instruction is used to obtain the bit fields.
The arguments for printf are stored in a0, a1, and a2, respectively.
Download C source from bitfields.zip.
01: #include <stdio.h>
02:
03: typedef union {
04: struct {
05: signed int immed:16;
06: unsigned int rt:5;
07: unsigned int rs:5;
08: unsigned int opcode:6;
09: };
10: unsigned int w;
11: } I_format_t;
12:
13:
14: int main()
15: {
16: I_format_t ia;
17: ia.w = 0xAFBE0010;
18: printf("\ninstruction: %X\n",ia.w);
19: printf("opcode: %X\n",ia.opcode);
20: printf("rs: %d rt: %d\n", ia.rs, ia.rt);
21: printf("immed: %d\n",ia.immed);
22: return 0;
23: }
The program was compiled with Microsoft C and then run, redirecting the output to a file:
>cl bitfield1.c >bitfield1 > bitfield1.txt
The resulting output file is included below
instruction: AFBE0010 opcode: 2B rs: 29 rt: 30 immed: 16
This instruction was decoded to sw s8,16(sp). See From C to Machine Instructions I.
The above code was modified slightly, as shown below, to run on the PIC32 Starter Kit. The output was the same.
--- C:\pic32\test\bitfield1.c ------------------------------------------------------------------ 1: #include2: #include "db_utils.h" 3: 4: typedef union { 5: struct { 6: signed int immed:16; 7: unsigned int rt:5; 8: unsigned int rs:5; 9: unsigned int opcode:6; 10: }; 11: unsigned int w; 12: } I_format_t; 13: 14: int main() 15: { 9D000018 27BDFFE0 addiu sp,sp,-32 9D00001C AFBF001C sw ra,28(sp) 9D000020 AFBE0018 sw s8,24(sp) 9D000024 03A0F021 addu s8,sp,zero 16: I_format_t ia; 17: ia.w = 0xAFBE0010; 9D000028 3C02AFBE lui v0,0xafbe 9D00002C 34420010 ori v0,v0,0x10 9D000030 AFC20010 sw v0,16(s8) 18: printf("\ninstruction: %X\n",ia.w); 9D000034 3C029D07 lui v0,0x9d07 9D000038 24443F18 addiu a0,v0,16152 9D00003C 8FC50010 lw a1,16(s8) 9D000040 0F400155 jal 0x9d000554 9D000044 00000000 nop 19: printf("opcode: %X\n",ia.opcode); 9D000048 8FC20010 lw v0,16(s8) 9D00004C 7C432E80 ext v1,v0,26,6 9D000050 3C029D07 lui v0,0x9d07 9D000054 24443F2C addiu a0,v0,16172 9D000058 00602821 addu a1,v1,zero 9D00005C 0F400155 jal 0x9d000554 9D000060 00000000 nop 20: printf("rs: %d rt: %d\n", ia.rs, ia.rt); 9D000064 8FC20010 lw v0,16(s8) 9D000068 7C432540 ext v1,v0,21,5 9D00006C 8FC20010 lw v0,16(s8) 9D000070 7C462400 ext a2,v0,16,5 9D000074 3C029D07 lui v0,0x9d07 9D000078 24443F38 addiu a0,v0,16184 9D00007C 00602821 addu a1,v1,zero 9D000080 0F400155 jal 0x9d000554 9D000084 00000000 nop 21: printf("immed: %d\n",ia.immed); 9D000088 87C30010 lh v1,16(s8) 9D00008C 3C029D07 lui v0,0x9d07 9D000090 24443F48 addiu a0,v0,16200 9D000094 00602821 addu a1,v1,zero 9D000098 0F400155 jal 0x9d000554 9D00009C 00000000 nop 22: DBPUTS("Program terminated. Click HALT and then RESET to stop the microcontroller. \n"); 9D0000A0 3C029D07 lui v0,0x9d07 9D0000A4 24443F54 addiu a0,v0,16212 9D0000A8 2405004C addiu a1,zero,76 9D0000AC 0F400034 jal 0x9d0000d0 9D0000B0 00000000 nop 23: return 0; 9D0000B4 00001021 addu v0,zero,zero 24: } 9D0000B8 03C0E821 addu sp,s8,zero 9D0000BC 8FBF001C lw ra,28(sp) 9D0000C0 8FBE0018 lw s8,24(sp) 9D0000C4 27BD0020 addiu sp,sp,32 9D0000C8 03E00008 jr ra 9D0000CC 00000000 nop
Maintained by John Loomis, updated Thu Aug 07 21:09:45 2008