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CSCI206 - Computer Organization &
Programming
Pseudo-Instructions and Addressing Data
zyBook: 5.8, 5.9
Developed and maintained by the Bucknell University Computer Science Department - 2017
Pseudo-instructions
What is a pseudo-instruction?
A pseudo-instruction is not a real MIPS instruction, but one
can use it as if it were.
For example, we know these two MIPS instructions
beq $t, $s, label
bne $t, $s, label
But we don’t have something like
blt $t, $s, label
Or the like
MIPS Pseudo-instructions
MIPS provides a set of such pseudo-instructions to make
programmers’ life easier.
Typically these pseudo-instructions are translated into real
instruction at the assembly time
For example,
blt $t, $s, label
Can be implemented as
slt $at, $t1, $t2
bne $at, $zero, label
One more example
li $t0, 0x8C828A43
Can be implemented as
lui $at, 0x8c82
ori $t0, $at, 0x8a43
[let’s do the worksheet]
Registers
Addressing data in a register
Encode the register number (0-31) in the
machine instruction
R/I formats
Immediates
Immediate data can be encoded in an I type
instruction
this data is implicitly addressed
what are the limitations?
32-bit Values from Immediates
•step 1: lui - load upper immediate
lui $t0, 0x00FF
•step 2: ori - or immediate
ori $t0, $t0, 0xFF00
•final result in $t0
0000 0000 1111 1111
which is 0x00FFFF00
1111 1111 0000 0000
MIPS optimizes for the common case, small immediate values
two instructions are handle the other (less common) cases.
Basic Program Memory Map
Local (automatic) variables inside functions;
activation records (recursion); unsafe to share
between functions / modules
Dynamically allocated memory; safe to
share between functions / modules
Global variables
Your program’s compiled machine code
Dynamic
(runtime)
allocation
Static
(compile time)
allocation
Base + displacement addressing
lw/lh/lb sw/sh/sb
Construct a memory address from a base address in a register plus some
immediate offset
Useful for variables
top of stack/heap in $sp/$gp register, immediate value selects offset
Useful for arrays
beginning of array in a register, immediate value selects the index
PC-Relative Addressing (branches)
PC is the
Program
Counter, it is
the address of
the instruction
being executed.
I-format instruction provides a 16 bit offset, relative to the next
instruction.
Good for short branches (local loops, if, case, etc).
Not enough bits for function calls (can’t jump very far away).
MIPS Branch Distance
The branch immediate address is a sign extended (2's
complement) value added to the current PC
Since we are dealing with instructions that must be
word aligned (32-bit), the last 2-bits are assumed to
be 00, therefore a branch can reach +0x1fffc or 0x20000 bytes!
(that is +32767 or -32768 instructions)
Pseudo-direct Addressing
J-type instructions contain a 26-bit word address
(unsigned)
(28-bit byte address), 2^28 == 256 MB range
This leaves the upper 4-bits unaccounted for
Uses the upper 4-bits of the next PC value
This is enough for most programs
How could you jump beyond the 256 MB?
MIPS
Addressing