MIPS Format Exceptions
I-format Exceptions
- Still have 2 registers and a constant value immediately present in the
instruction.
-
rs: operand or base address (5 bits)
-
rt: operand or data register (5 bits)
-
immediate: value or offset (16 bits)
| Name |
Format |
Layout |
Example |
| 6 bits |
5 bits |
5 bits |
5 bits |
5 bits |
6 bits |
| op |
rs |
rt |
immediate |
| beq | I | 4 |
1 | 2 | 25 (offset) |
beq $1, $2, 100 |
| bne | I | 5 |
1 | 2 | 25 (offset) |
bne $1, $2, 100 |
| lui | I | 15 |
0 | 1 | 100 |
lui $1, 100 |
| lw | I | 35 |
2 | 1 | 100 (offset) |
lw $1, 100($2) |
| sw | I | 43 |
2 | 1 | 100 (offset) |
sw $1, 100($2) |
lw $t0, 32($s3) (registers 8 and 19)
| op |
rs |
rt |
immediate |
35 |
19 |
8 |
32 |
100011 |
10011 |
01000 |
0000000000100000 |
lui $t0, 1028 (register 8)
| op |
rs |
rt |
immediate |
15 |
0 |
8 |
1028 |
001111 |
00000 |
01000 |
0000010000000100 |
beq $t0, $zero, ENDIF
The offset stored in a beq (or bne)
instruction is the number of
instructions from the PC (the instruction after the beq
instruction) to the label (ENDIF in this example). Or, in
terms of addresses, it is the difference between the address associated
with the label and the PC, divided by four.
offset = (addrFromLabelTable - PC) / 4
In the example above, if the beq instruction is at address
1004, and thus the PC is 1008, and if
ENDIF is at address 1028, then the value
stored in the machine instruction would be
offset = (1028 - 1008) / 4 = 5
| op |
rs |
rt |
immediate |
4 |
8 |
0 |
5 |
000100 |
01000 |
00000 |
0000000000000101 |
R-format Exceptions
- Still have opcode
0 (all of them!), 3 registers, a shift
amount, and a funct code.
| Name |
Format |
Layout |
Example |
| 6 bits |
5 bits |
5 bits |
5 bits |
5 bits |
6 bits |
| op |
rs |
rt |
rd |
shamt |
funct |
| sll | R | 0 |
0 | 2 | 1 | 10 |
0 | sll $1, $2, 10 |
| srl | R | 0 |
0 | 2 | 1 | 10 |
2 | srl $1, $2, 10 |
| jr | R | 0 |
31 | 0 | 0 | 0 |
8 | jr $31 |
NOTE: op is 0, so funct disambiguates
srl $s0, $s1, 1 (registers 16, 17)
| op |
rs |
rt |
rd |
shamt |
funct |
0 |
0 |
17 |
16 |
1 |
2 |
000000 |
00000 |
10001 |
10000 |
00001 |
000010 |
jr $ra (register 31)
| op |
rs |
rt |
rd |
shamt |
funct |
0 |
31 |
0 |
0 |
0 |
8 |
000000 |
11111 |
00000 |
00000 |
00000 |
001000 |
Alyce Brady, Kalamazoo College