MIPS Instructions
J-type Instruction Encoding
• We have jump and jump-and-link instructions
– Note that we have 26 bits for the target fields, which
represents the number of instructions (words) instead of
bytes
• In terms of bytes, it represents 28 bits in terms of bytes
– But PC requires 32 bits
• Where do we get the other 4 bits?
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J-type Instruction Encoding
• Pseudo-direct addressing
– In J-type instructions, the jump address is formed
by upper 4 bits of the current PC, 26 bits of the
target address field in the instruction, and two bits
of 0’s
• What is the largest program a J-type instruction works
properly?
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Peek into the Future
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Jump Register Instruction
• Jump register (jr)
– Unconditionally jump to the address given by register rs
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Big Immediates
• We know that we can use 16-bit immediate numbers
in MIPS instructions such as addi
– What about large numbers?
• We also need to load 32 bit addresses in order to use
jr
– Load address (la)
– Load the address of a label into the register (note: not the
content of the address)
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MIPS Addressing for 32-bit Immediates
• In MIPS, the immediate field has 16 bits
– In order to handle 32-bit immediate operands,
the MIPS includes load upper immediate (lui)
• Which sets the upper 16 bits of a constant in a
register and fills the lower 16 bits with 0’s
• Then one can set the lower 16 bits using ori
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Example
• To load the following 32-bit constant in $s0,
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 1 1 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0
– We first need to do
lui $s0, 61
• Why?
– Then we need to do
ori $s0, $s0, 35072
• Why?
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Branching Far Away
• Note that for conditional branch instructions the
offset is 16 bits
– What can we do if the branch is further away than the 16bit offset can represent?
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Branching Far Away
• Note that for conditional branch instructions the
offset is 16 bits
– What can we do if the branch is further away than the 16bit offset can represent?
– We can replace it using a pair of instructions
bne $s0, $s1, L2
j L1
L2:
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MIPS Assembly Instructions
add
MIPS assembly language
Example
Meaning
add $s1, $s2, $s3
$s1 = $s2 + $s3
Three operands; data in registers
subtract
sub $s1, $s2, $s3
$s1 = $s2 - $s3
Three operands; data in registers
$s1 = $s2 + 100
$s1 = Memory[$s2 + 100]
Memory[$s2 + 100] = $s1
$s1 = Memory[$s2 + 100]
Memory[$s2 + 100] = $s1
Used to add constants
Category
Arithmetic
Instruction
addi $s1, $s2, 100
load word
lw $s1, 100($s2)
store word
sw $s1, 100($s2)
load byte
lb $s1, 100($s2)
store byte
sb $s1, 100($s2)
load upper immediate lui $s1, 100
add immediate
Data transfer
Conditional
branch
Unconditional jump
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$s1 = 100 * 2
Comments
Word from memory to register
Word from register to memory
Byte from memory to register
Byte from register to memory
Loads constant in upper 16 bits
branch on equal
beq
$s1, $s2, 25
if ($s1 == $s2) go to
PC + 4 + 100
Equal test; PC-relative branch
branch on not equal
bne
$s1, $s2, 25
if ($s1 != $s2) go to
PC + 4 + 100
Not equal test; PC-relative
set on less than
slt
$s1, $s2, $s3
if ($s2 < $s3) $s1 = 1;
else $s1 = 0
Compare less than; for beq, bne
set less than
immediate
slti
jump
j
jr
jal
jump register
jump and link
$s1, $s2, 100 if ($s2 < 100) $s1 = 1;
Compare less than constant
else $s1 = 0
2500
$ra
2500
go to 10000
Jump to target address
go to $ra
For switch, procedure return
$ra = PC + 4; go to 10000 For procedure call
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Procedures and Functions
• We programmers use procedures and
functions to structure and organize programs
– To make them easier to understand
– To allow code to be reused
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A Simple Example
Note that there are steps involved in the calling
function as well as in the one being called
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MIPS Calling Conventions
• MIPS assembly follows the following convention in
using registers
– $a0 - $a3: four argument registers in which to pass
parameters
– $v0 - $v1: two value registers in which to return values
– $ra: one return address register to return to the point of
origin
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Registers
• Remember that the same registers are used by
both the caller and callee
– What do we need to do in order to guarantee the
correctness of the program?
• For the following code for example, what do we need to
do?
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Register Spilling
• The callee has to save all the registers it uses
and restore the values before it returns
– By storing them on the stack
• At the beginning
– Then restoring them
• At the end
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Simple Example
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The Stack Pointer
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MIPS Calling Conventions - more
• MIPS software divides 18 of the registers into
two groups
– $t0 - $t9: 10 temporary registers that are not
preserved by the callee on a procedure call
• These are caller-saved registers since the caller must
save the ones it is using
– $s0 - $s7: 8 saved registers that must be preserved
on a procedure call
• These are callee-saved registers since the callee must
save the ones it uses
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Caller Must Do
• Before it calls a procedure/function, it must
– Pass parameters
• Up to four parameters are passed by $a0 - $a3
– Save caller-saved registers on the stack
• It includes $a0 - $a3 (since the callee may change these
values), $s0 - $s9, and $ra
• Why $ra?
– Execute a jal instruction, which jumps to the
callee’s first instruction and save the next
instruction in $ra
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Caller Must Do – cont.
• After the procedure/function call, it needs to
– Read the returned values from $v0 and $v1
– Restore caller-saved registers
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Callee Must Do
• Before it does its calculations, the callee must do the
following
– Allocate memory for its frame by subtracting its frame size
from the stack pointer
– Save callee-saved registers in the frame
• It must save $s0 - $s7, $fp, $ra before changing them
• $ra needs to be saved if the callee itself makes a call
– When needed, establish the frame pointer $fp by loading
$sp to it
• In this case, $fp must be saved
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Allocating Space for Local Data on Stack
• In MIPS, local variables are also stored on the stack
– The segment of the stack containing a procedure’s saved
registers and local variables is called a procedure frame
(or activation record)
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Callee Must Do
• After its calculations and before it returns to the
caller, the callee must
– Place the return values in $v0 and $v1 (if needed)
– Restore all the callee-saved registers that were saved at
the procedure entry
– Pop the stack frame by adding the frame size to $sp
– Return by jumping to the address in register $ra using jr
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Earlier Example
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Revised Version
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