1. MIPS Instructions

2. J-type Instruction Encoding
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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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3. J-type Instruction Encoding
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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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4. 12/1/2018
Peek into the Future
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5. Jump Register Instruction
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Jump Register Instruction
Jump register (jr)
Unconditionally jump to the address given by register rs
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6. 12/1/2018
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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7. MIPS Addressing for 32-bit Immediates
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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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8. Example To load the following 32-bit constant in $s0,
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Example
To load the following 32-bit constant in $s0,
We first need to do
lui $s0, 61
Why?
Then we need to do
ori $s0, $s0, 35072 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
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9. 12/1/2018
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?
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10. 12/1/2018
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 $s0, $s1, L2
j L1
L2:
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11. MIPS Assembly Instructions
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MIPS Assembly Instructions
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12. Procedures and Functions
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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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13. 12/1/2018
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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14. MIPS Calling Conventions
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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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15. 12/1/2018
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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16. 12/1/2018
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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17. 12/1/2018
Simple Example
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18. The Stack Pointer 12/1/2018 12/1/2018 week04-3.ppt
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19. MIPS Calling Conventions - more
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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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20. Caller Must Do Before it calls a procedure/function, it must
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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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21. Caller Must Do – cont. After the procedure/function call, it needs to
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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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22. 12/1/2018
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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23. Allocating Space for Local Data on Stack
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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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24. 12/1/2018
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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25. 12/1/2018
Earlier Example
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26. 12/1/2018
Revised Version
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