[PDF] Functions in MIPS Oops in Strings/pointers (example

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1

Lectures 5

ƒAnnouncements:

ƒToday:

³Oops in Strings/pointers (example from last time)

³Functions in MIPS

2 int foo(char *s) { int L = 0; while (*s++) { ++L; return L;

OOPS -What does this C code do?

3

An Example Function: Factorial

int fact(int n) {fact: li $t0, 1 int i, f = 1; move $t1,$a0 # set i to n for (i = n; i > 0; i--) loop: f = f * i; blez $t1,exit # exit if done return f; mul $t0,$t0,$t1 # build factorial } addi $t1, $t1,-1 # i-- j loop exit: move $v0,$t0 jr $ra 4

Register Correspondences

ƒ$zero $0 Zero

ƒ$at $1 Assembler temp

ƒ$v0-$v1 $2-3 Value (return from function)

ƒ$a0-$a3 $4-7 Argument (to function)

ƒ$t0-$t7 $8-15 Temporaries

ƒ$s0-$s7 $16-23 Saved Temporaries Saved

ƒ$t8-$t9 $24-25 Temporaries

ƒ$k0-$k1 $26-27 Kernel (OS) Registers

ƒ$gp $28 Global Pointer Saved

ƒ$sp $29 Stack Pointer Saved

ƒ$fp $30 Frame Pointer Saved

ƒ$ra $31 Return Address Saved

5

Functions in MIPS

ƒJH·OO PMON MNRXP POH 3 VPHSV LQ OMQGOLQJ IXQŃPLRQ ŃMOOV

1.7OH SURJUMP·V IORR RI ŃRQPURO PXVP NH ŃOMQJHGB

2.Arguments and return values are passed back and forth.

3.Local variables can be allocated and destroyed.

ƒAnd how they are handled in MIPS:

³New instructions for calling functions.

³Conventions for sharing registers between functions.

³Use of a stack.

6

Control flow in C

ƒInvoking a function changes the

control flow of a program twice.

1.Calling the function

2.Returningfrom the function

ƒIn this example the mainfunction

calls facttwice, and fact returns twice³but to differentlocations in main.

ƒEach time fact is called, the CPU

has to remember the appropriate return address.

ƒNotice that main itself is also a

function! It is called by the operating system when you run the program. int main() t1 = fact(8); t2 = fact(3); t3 = t1 + t2; int fact(int n) int i, f = 1; for (i = n; i > 1; i--) f = f * i; return f; 7

Function control flow MIPS

ƒMIPS uses the jump-and-link instruction jalto call functions. ³The jal saves the return address (the address of the nextinstruction) in the dedicated register $ra, before jumping to the function. ³jal is the only MIPS instruction that can access the value of the program counter, so it can store the return address PC+4 in $ra. jal Fact ƒTo transfer control back to the caller, the function just has to jump to the address that was stored in $ra. jr $ra ƒIHP·V QRR MGG POH ÓMO MQG ÓU LQVPUXŃPLRQV POMP MUH QHŃHVVMU\ IRU RXU factorial example. 8

Data flow in C

ƒFunctions accept argumentsand

produce return values.

ƒThe blueparts of the program

show the parameters and arguments of the fact function.

ƒThe purpleparts of the code deal

with returning and using a result. int main() t1 =fact(8); t2 =fact(3); t3 = t1 + t2; intfact(int n) int i, f = 1; for (i = n; i > 1; i--) f = f * i; return f; 9

Data flow in MIPS

ƒMIPS uses the following conventions for function arguments and results. argument registers $a0-$a3before calling the function with jal. $v0-$v1, before returning via jr. ƒThese conventions are not enforced by the hardware or assembler, but programmers agree to them so functions written by different people can interface with each other. ƒIMPHU RH·OO PMON MNRXP OMQGOLQJ MGGLPLRQMO MUJXPHQPV RU UHPXUQ YMOXHVB 10 ƒAssembly language is untyped³there is no distinction between integers, characters, pointers or other kinds of values. ƒIt is up to youPR ´P\SH ŃOHŃNµ \RXU SURJUMPVB HQ SMUPLŃXOMU PMNH VXUH your function arguments and return values are used consistently. ƒFor example, what happens if somebody passes the addressof an integer (instead of the integer itself) to the fact function?

A note about types

11

The big problem so far

ƒThere is a big problem here!

³The main code uses $t1to store the result of fact(8).

³But $t1is also used within the fact function!

ƒThe subsequent call to fact(3) will overwrite the value of fact(8) that was stored in $t1. 12 A:... 3XP %·V MUJV LQ M0-$a3 jalB# $ra = A2

A2:...

B:... 3XP F·V MUJV LQ M0-$a3, HUMVLQJ %·V MUJVA jalC# $ra = B2

B2:...

jr$ra# Where does # this go??? C:... jr$ra

Nested functions

ƒA similar situation happens when

you call a function that then calls another function. ƒIHP·V VM\ $ ŃMOOV % ROLŃO ŃMOOV FB quotesdbs_dbs21.pdfusesText_27

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