Recitation Session 3
CSE341 Computer Organization
University at Buffalo
radhakri@buffalo.edu
February 19, 2016
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Recitation Session 3
1/21
Assembler code nrfact.asm Project 1
You need to get familiarized with several concepts before venturing
into Project 1
CSE341 Computer Organization
Recitation Session 3
2/21
Procedure Calls in MIPS
You can think of a procedure like a spy who leaves with a secret
plan, acquires resources, performs the task, covers his or her tracks,
and then returns to the point of origin with the desired result
1
Put parameters in a place where the procedure can access
them.
2
Transfer control to the procedure.
3
Acquire the storage resources needed for the procedure.
4
Perform the desired task.
5
Put the result value in a place where the calling program can
access it.
6
Return control to the point of origin, since a procedure can be
called from several points in a program.
CSE341 Computer Organization
Recitation Session 3
3/21
Procedure Calls in MIPS
$a0-$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
MIPS assembly language includes an instruction just for the
procedures: it jumps to an address and simultaneously saves the
address of the following instruction in register $ra. The
jump-and-link instruction (jal) is simply written – jal Procedure
What instruction do you execute return to original location after
completing procedure?
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Recitation Session 3
4/21
Stack Pointer in MIPS
Useful when a compiler needs more registers for a procedure
than the four argument and two return value registers
The stack pointer is adjusted by one word for each register
that is saved or restored.
placing data onto the stack is called a push, and removing
data from the stack is called a pop
stacks grow from higher addresses to lower addresses. This
convention means that you push values onto the stack by
subtracting from the stack pointer. Adding to the stack
pointer shrinks the stack, thereby popping values off the stack.
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Recitation Session 3
5/21
Memory Layout - MIPS Assembly
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Recitation Session 3
6/21
Addressing in Branches and Jumps
The MIPS jump instructions have the simplest addressing. They
use the final MIPS instruction format, called the J-type, which
consists of 6 bits for the operation field and the rest of the bits for
the address field.
j 10000 # go to location 10000
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Recitation Session 3
7/21
Addressing in Branches and Jumps
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Recitation Session 3
8/21
Addressing in Branches and Jumps - Further Reading
Read through these concepts in the text and familiarize yourself
with different addressing schemes
Immediate addressing, where the operand is a constant
within the instruction itself
Register addressing, where the operand is a register
Base or displacement addressing, where the operand is at
the memory location whose address is the sum of a register
and a constant in the instruction
PC-relative addressing, where the branch address is the sum
of the PC and a constant in the instruction
Pseudodirect addressing, where the jump address is the 26
bits of the instruction concatenated with the upper bits of the
PC
CSE341 Computer Organization
Recitation Session 3
9/21
Reading Assembly Programs
Names that begin with a period like .data and .globl, are
assembler directives that tell the assembler how to translate a
program but do not produce machine instructions
Names followed by a colon, such as str: or main:, are labels
that name the next memory location
If a line begins with a label, the assembler records in its
symbol table the name of the label and the address of the
memory word that the instruction occupies
The .data directive tells the assembler to store the string in
the programs data segment, and the .text directive tells the
assembler to store the instructions in its text segment.
CSE341 Computer Organization
Recitation Session 3
10/21
Reading Assembly Programs
Some assemblers also implement pseudoinstructions, which
are instructions provided by an assembler but not
implemented in hardware.
Many pseudoinstructions could also be simulated with macros,
but the MIPS assembler can generate better code for these
instructions because it can use a dedicated register ($at) and
is able to optimize the generated code.
CSE341 Computer Organization
Recitation Session 3
11/21
SPIM System Calls
SPIM provides a small set of operating systemlike services
through the system call (syscall) instruction.
To request a service, a program loads the system call code
into register $v0 and arguments into registers $a0$a3 (or $f12
for floating-point values)
System calls that return values put their results in register $v0
(or $f0 for floating-point results).
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Recitation Session 3
12/21
SPIM System Calls
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Recitation Session 3
13/21
SPIM System Calls
What does this Program print?
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Recitation Session 3
14/21
String Copy Example
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15/21
String Copy Example
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Recitation Session 3
16/21
String Copy Example
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Recitation Session 3
17/21
String Copy Example
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Recitation Session 3
18/21
String Copy Example
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Recitation Session 3
19/21
Swap Integer Example
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Recitation Session 3
20/21
Swap Integer Example
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Recitation Session 3
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