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CS/ECE 354 Machine Organization and Programming
Fall 2008 Section 1
Instructor
Mark D. Hill
TAs
Mark Liu &
Somayeh Sardashti
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MIPS RISC Conventions
Register Usage Conventions
Purpose
The purpose of this document is to clarify the
procedure/register usage conventions you are expected
to follow to get the most credit for the program that implements
functions/procedures.
Class has not
presented the MIPS RISC conventions for procedure
linkage and register usage exactly as they are stated in the
MIPS RISC Architecture manual. So, what follows are
not MIPS RISC conventions at all.
Conventions to Follow
- Correct usage of registers.
All code may use registers $8-$25.
All of registers $8-$25 are designated as callee (child) saved.
Therefore, before any function uses a register,
it must save the value (on the stack, in the function's
activation record).
And, before returning, the function must restore the
saved values.
- Parameter location.
All parameters are passed on the stack.
The parent allocates activation record space for any and all
parameters,
and
places the actual parameters into that activation record
space.
The child retrieves and uses the formal arguments.
- Size of activation record.
There are two choices for the size of a procedure's activation
record. A compiler would either allocate an activation record
that is the same size for all functions (and it would contain
extra, unused space for many functions), or it would allocate
an activation record that is exactly the correct size for
a function. This size would be different for different functions.
Make activiation records the exact size needed for the function.
Here is a diagram indicating the contents of activation records.
Note that the ordering of data within the activation record
will be the same for all functions.
Here are activation records for functions A and B.
Assume that function A calls function B, and passes 5 parameters.
Assume that function B calls function C, and passes 1 parameter.
| | smallest addresses
|----------------|
| |
|================|
| P1 (B passes |---
| to C) | |
|----------------| |
| any regs | |
| used by B: $8 | |
|----------------| |
| $9 | |
|----------------| |---- function B's activation record
| $10 | |
|----------------| |
| etc.| |
|----------------| |
| copy of $ra | |
| B's return addr| --|
|================|
| P1 (A passes |---
| to B) | |
|----------------| |
| P2 (A passes | |
| to B) | |
|----------------| |
| P3 (A passes | |
| to B) | |
|----------------| |
| P4 (A passes | |
| to B) | |
|----------------| |
| P5 (A passes | |
| to B) | |
|----------------| |---- function A's activation record
| any regs | |
| used by A: $8 | |
|----------------| |
| $9 | |
|----------------| |
| $10 | |
|----------------| |
| etc.| |
|----------------| |
| copy of $ra | |
| A's return addr| --|
|================|
| |
|----------------|
| |
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