Bitfield Example 2

This example extends the program in example 1 to decode both I-format and R-format instructions. A union is used to give dual assignments for the least-signficant 16 bits of the instruction. See From C to Machine Instructions I from which the test instructions were taken.

C Source

C source files: bitfields.zip

01: #include <stdio.h>
02: 
03: typedef union {
04:         struct {
05:                 unsigned int fn:6;
06:                 unsigned int sh:5;
07:                 unsigned int rd:5;
08:         };
09:         struct {
10:                 signed int immed:16;
11:                 unsigned int rt:5;
12:                 unsigned int rs:5;
13:                 unsigned int opcode:6;
14:         };
15:         unsigned int w;
16: } mips_format_t;
17: 
18: 
19: int main()
20: {
21:         int i;
22:         mips_format_t ia;
23:         unsigned int codes[] = {0x27bdffe8, 0xAFBE0010, 0x03A0F021, 0x2402000E};
24:         int n = sizeof(codes)/sizeof(int);
25:         for (i=0; i<n; i++) {
26:                 ia.w = codes[i];
27:                 printf("\ninstruction: %X\n",ia.w);
28:                 printf("opcode: %X\n",ia.opcode);
29:                 printf("rs: %d rt: %d\n", ia.rs, ia.rt);
30:                 if (ia.opcode==0) {
31:                         printf("rd: %d  sh: %d  fn: %X\n",ia.rd,ia.sh,ia.fn);
32:                 }
33:                 else {
34:                         printf("immed: %d\n",ia.immed);
35:                 }
36:         }
37:         return 0;
38: }

Line 24, shown below, determines the length of the codes array

         int n = sizeof(codes)/sizeof(int);

instruction: 27BDFFE8
opcode: 9
rs: 29 rt: 29
immed: -24

instruction: AFBE0010
opcode: 2B
rs: 29 rt: 30
immed: 16

instruction: 3A0F021
opcode: 0
rs: 29 rt: 0
rd: 30  sh: 0  fn: 21

instruction: 2402000E
opcode: 9
rs: 0 rt: 2
immed: 14