The following describes a function call abstraction for the MIPS architecture. This may be slightly different from the one you will use for the project.
MIPS uses the register
$sp as the stack pointer and the
$fp as the frame pointer. In the following MIPS code
we use both a dynamic link and a static link embedded in the
Consider the previous program:
procedure P ( c: integer ) x: integer; procedure Q ( a, b: integer ) i, j: integer; begin x := x+a+j; end; begin Q(x,c); end;
The activation record for
P sees it) is shown in the first figure below:
The activation record for
Q sees it) is shown in the second figure above.
The third figure shows the structure of the run-time stack at the point where
x := x+a+j is executed. This statement uses
x, which is defined
P. We can't assume that
P, so we should not use
the dynamic link to retrieve
x; instead, we need to use the static link,
which points to the most recent activation record of
Thus, the value of variable
x is computed by:
lw $t0, -8($fp) # follow the static link of Q lw $t1, -12($t0) # x has offset=-12 inside PFunction/procedure arguments are pushed in the stack before the function call. If this is a function, then an empty placeholder (4 bytes) should be pushed in the stack before the function call; this will hold the result of the function.
Each procedure/function should begin with the following code (prologue):
sw $fp, ($sp) # push old frame pointer (dynamic link) move $fp, $sp # frame pointer now points to the top of stack subu $sp, $sp, 500 # allocate say 500 bytes in the stack # (for frame size = 500) sw $ra, -4($fp) # save return address in frame sw $v0, -8($fp) # save static link in frame(where
$v0is set by the caller - see below) and should end with the following code (epilogue):
lw $ra, -4($fp) # restore return address move $sp, $fp # pop frame lw $fp, ($fp) # restore old frame pointer (follow dynamic link) jr $ra # returnFor each procedure call, you need to push the arguments into the stack and set
$v0to be the right static link (very often it is equal to the static link of the current procedure; otherwise, you need to follow the static link a number of times). For example, the call
Pis translated into:
lw $t0, -12($fp) sw $t0, ($sp) # push x subu $sp, $sp, 4 lw $t0, 4($fp) sw $t0, ($sp) # push c subu $sp, $sp, 4 move $v0, $fp # load static link in $v0 jal Q # call procedure Q addu $sp, $sp, 8 # pop stackNote that there are two different cases for setting the static link before a procedure call. Lets say that caller_level and callee_level are the nesting levels of the caller and the callee procedures (recall that the nesting level of a top-level procedure is 0, while the nesting level of a nested procedure embedded inside another procedure with nesting level l, is l + 1). When the callee is lexically inside the caller's body, that is, when callee_level=caller_level+1, we have:
move $v0, $fpThe call
Pis such a case because the nesting levels of
Qare 0 and 1, respectively. Otherwise, we follow the static link of the caller d + 1 times, where d=caller_level-callee_level (the difference between the nesting level of the caller from that of the callee). For d=0, that is, when both caller and callee are at the same level, we have
lw $v0, -8($fp)For d=2 we have
lw $t1, -8($fp) lw $t1, -8($t1) lw $v0, -8($t1)These cases are shown in the following figure:
Note also that, if you have a call to a function, you need to allocate 4 more bytes in the stack to hold the result.
See the factorial example for a concrete example of a function expressed in MIPS code.