MIPS Instructions Jtype Instruction Encoding We have jump

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? 9/2/2021 week 04 -3. ppt 2

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? 9/2/2021 week 04 -3. ppt 3

Peek into the Future 9/2/2021 week 04 -3. ppt 4

Jump Register Instruction • Jump register (jr) – Unconditionally jump to the address given by register rs 9/2/2021 week 04 -3. ppt 5

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) 9/2/2021 week 04 -3. ppt 6

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 9/2/2021 week 04 -3. ppt 7

Example • To load the following 32 -bit constant in $s 0, 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 1 1 0 0 0 1 0 0 0 0 – We first need to do lui $s 0, 61 • Why? – Then we need to do ori $s 0, 35072 • Why? 9/2/2021 week 04 -3. ppt 8

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 16 bit offset can represent? 9/2/2021 week 04 -3. ppt 9

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 16 bit offset can represent? – We can replace it using a pair of instructions bne $s 0, $s 1, L 2 j L 1 L 2: 9/2/2021 week 04 -3. ppt 10

MIPS Assembly Instructions 9/2/2021 week 04 -3. ppt 11

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 9/2/2021 week 04 -3. ppt 12

A Simple Example Note that there are steps involved in the calling function as well as in the one being called 9/2/2021 week 04 -3. ppt 13

MIPS Calling Conventions • MIPS assembly follows the following convention in using registers – $a 0 - $a 3: four argument registers in which to pass parameters – $v 0 - $v 1: two value registers in which to return values – $ra: one return address register to return to the point of origin 9/2/2021 week 04 -3. ppt 14

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? 9/2/2021 week 04 -3. ppt 15

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 9/2/2021 week 04 -3. ppt 16

Simple Example 9/2/2021 week 04 -3. ppt 17

The Stack Pointer 9/2/2021 week 04 -3. ppt 18

MIPS Calling Conventions - more • MIPS software divides 18 of the registers into two groups – $t 0 - $t 9: 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 – $s 0 - $s 7: 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 9/2/2021 week 04 -3. ppt 19

Caller Must Do • Before it calls a procedure/function, it must – Pass parameters • Up to four parameters are passed by $a 0 - $a 3 – Save caller-saved registers on the stack • It includes $a 0 - $a 3 (since the callee may change these values), $s 0 - $s 9, and $ra • Why $ra? – Execute a jal instruction, which jumps to the callee’s first instruction and save the next instruction in $ra 9/2/2021 week 04 -3. ppt 20

Caller Must Do – cont. • After the procedure/function call, it needs to – Read the returned values from $v 0 and $v 1 – Restore caller-saved registers 9/2/2021 week 04 -3. ppt 21

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 $s 0 - $s 7, $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 9/2/2021 week 04 -3. ppt 22

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) 9/2/2021 week 04 -3. ppt 23

Callee Must Do • After its calculations and before it returns to the caller, the callee must – Place the return values in $v 0 and $v 1 (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 9/2/2021 week 04 -3. ppt 24

Earlier Example 9/2/2021 week 04 -3. ppt 25

Revised Version 9/2/2021 week 04 -3. ppt 26