Function calls.ppt

7/27/2019 Function calls.ppt

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Assembly Program Layout

Function Call Overview

Parameter and Result Passing

Function Variables

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Recall Hello World Program#include "QTerm.h" // Accessing the QTerm-J10 Terminal

#include "serial.h" // Accessing the serial ports

#include "PPC_Support.h" // msleep & miscellaneous functions

main ()

{

LCD_Init();

LCD_PutString("Hello World!\r");

return 0;

}

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Hello World in Assembly.export main ; Tell the linker other files may need to

; see this function

.extern LCD_Init ; Tell the linker to import functions

.extern LCD_PutString ; in other files

.text

main: ; main function code

mflr r0 ; move LR into r0

stw r0,4(rsp) ; store LR into stack

stwu rsp,-8(rsp) ; create 8-byte stack space

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directive for text (code) segment

label


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Hello World in Assemblybl LCD_Init ; call LCD_Initlis r3,mystring@ha ; load mystring upper halfaddi r3,r3,mystring@l ; load mystring lower halfbl LCD_PutString ; call LCD_PutString

li r3,0 ; clear r3addi rsp,rsp,8 ; restore stack pointerlwz r0,4(rsp) ; load original LR value into r0mtlr r0 ; move r0 value into LRblr ; end of main

.data

mystring: .ascii "Hello World!\r"

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label

directive for data segment


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Passing Parameters and ResultsR0 Volatile Language SpecificR1 Dedicated Stack Pointer (SP)

R2 Dedicated Read-only small data area anchor

R3 -R4 Volatile Parameter Passing / return values

R5 - R10 Volatile Parameter Passing

R11 - R12 Volatile Read-write small data area anchorR13 Dedicated

R14 R31 Nonvolatile Language specific

F0 Volatile Parameter passing/return values

F1 Volatile Parameter passing

F2 F8 Volatile

F9 F13 Volatile

F14 F31 Nonvolatile

Fields CR2 CR4 Nonvolatile

Other CR Fields Volatile

LR, XER, CTR,

FPSCRVolatile

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EABI Register Usage Conventions

EABI: Embedded Application Binary Interface


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Passing Parameters and Resultint max(int x, int y){

int max;

if(x > y)max = x;

elsemax = y;

return max;}

intmax3(int x, int y, int z){

return max(max(x, y), z);}

.text

max:cmpw r3, r4 ; x is r3, y is r4

ble y_greaterb x_greater

y_greater:mr r3,r4 ; max is r3 = y

x_greater:blr ; max is r3 = x

; mr: move register (pseudo); mr r3, r4 addi r3, r4, 0

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What registers are used for x and y?

Where is the returning result?


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Passing Parameters and Result(continuing)max3:

mflr r0 ; discuss laterstw r0,4(rsp) ;stwu rsp,-16(rsp) ;

stw r31,12(rsp) ;mr r31,r5 ; save z to r31; r31 is nonvolatilebl max ; call maxmr r4,r31 ; now r3=max(x,y), r4=zbl max ; call maxlwz r31,12(rsp) ; discuss later

addi rsp,rsp,16 ;lwz r0,4(rsp) ;mtlr r0; ;blr ; return; now r3 stores max(x, y, z)

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Nested Function Callsmax3:mflr r0 ; save LR to r0stw r0,4(rsp) ; now LR saved into stackstwu rsp,-16(rsp) ; create 16 bytes in stack AND save old SP

stw r31,12(rsp) ; save nonvolatile r31 into stackmr r31,r5 ;bl max ; call maxmr r4,r31 ;bl max ; call max

lwz r31,12(rsp) ; restore r31 valueaddi rsp,rsp,16 ; release 16 byte from stack AND restore SPlwz r0,4(rsp) ; restore old LR valuemtlr r0 ; move to LRblr ; return

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Nested Function Callsint max(int x, int y)

Which areas are saved instack?

int max3(int x, int y, int z)

Which areas are saved instack?

Exercise: Give the layout ofmax3 stack usage

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FPR Save Area

(optional)

GPR Save Area

(optional)CR Save Area

(optional)

Local Variables Area

(optional)

Padding to 8-byte boundary

(optional)

LR Save Word

Back Chain (SP Save) Word

EABI stack usageHigh-end address

Load-end address


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Function Exercisesvoid add_array (int X[], int Y[], int N)

{

int i;for (i = 0; i < N; i++)

X[i] += Y[i];

}

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add_array.c

Write add_array.asm to replace this program


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Function Call and ReturnControl flow transfercaller_func: // caller address

bl caller_func // call callee_funcblr // return

callee_func:blr // return to caller

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Function Call and ReturnControl flow transfer

LR Link register, saving the return address

bl func_label Jump to func_label, saving the returnaddress in LR

blr Jump to the return address in LR

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Function Call and ReturnMany other issues:

Nested function call/return

Parameter and result passing Register usage

Stack usage and frame format

PowerPC EABI

Embedded Application BinaryInterface

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Parameters and Result PassingRegister R3-R10: Parameters

Register R3-R4: Results

Why not use stack only?

When to use stack?

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Parameters and Result Passingint max(int x, int y){int max;

if (x > y)max = x;

elsemax = y;

return max;}

int max3(int x, int y, int z){

return max(max(x, y), z);} 15


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Parameters and Result Passing.text; reg usage: r3 x, r4 ymax:

cmpw r3, r4 ; x is r3, y is r4ble x_greatery_greater:mr r3,r4 ; max is r3 = yx_greater:blr ; max is r3 = x

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Parameters and Result Passingmax3:; prologue (see later)

; reg usage: x r3, y r4, z r5

mr r31,r5 ; save z to r31 (nonvolatile)bl max ; call max

mr r4,r31 ; now r3=max(x,y), r4=z

bl max ; call max

; epilogue (see later)

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Stack Frame

Stack top grows downwards Stack frame created during function prologue

Stack frame Released during function epilogue

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Stack Frame EABI Stack usage To save nonvolatile

registers

To store local variables

To pass extra parametersand return values

To store return addressand old stack top

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FPR Save Area

(optional)

GPR Save Area

(optional)

CR Save Area(optional)

Local Variables Area

(optional)

Padding to 8-byte boundary

(optional)

LR Save Word


High-end address

Load-end address

Function parameters

(optional)


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Stack FrameR0 Volatile Language Specific

R1 Dedicated Stack Pointer (SP)

R2 Dedicated Read-only small data area anchor

R3 -R4 Volatile Parameter Passing / return values

R5 - R10 Volatile Parameter Passing

R11 - R12 Volatile Read-write small data area anchor

R13 Dedicated

R14 R31 Nonvolatile Language specificF0 Volatile Parameter passing/return values

F1 Volatile Parameter passing

F2 F8 Volatile

F9 F13 Volatile

F14 F31 Nonvolatile

Fields CR2 CR4 NonvolatileOther CR Fields Volatile

LR, XER, CTR, FPSCR Volatile

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EABI Register Usage Conventions


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Stack FrameWhat is the stack frame for this function body?

; prologue (see later)

mr r31,r5 ; save z to r31 (nonvolatile)bl max ; call max

mr r4,r31 ; now r3=max(x,y), r4=z

bl max ; call max

; epilogue (see later)

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Stack FrameStack Frame of Max3

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r31 save

LR Save Word (not used)


SP

0(rsp)

4(rsp)

12(rsp)

Padding8(rsp)

LR Save Word


old SP (rsp)

max3 frame

the frame of

max3s caller

Note: A function uses its callers LR save word to

save the return address.

4(rsp)


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Function Prologue/EpiloguePrologue for max3

max3:

mflr r0 ; save LR to r0stw r0,4(rsp) ; then to stack

stwu rsp,-16(rsp) ; create frame

stw r31,12(rsp) ; save r31

; r31 will be used to hold z

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Function Prologue/EpiloguePrologue for max3

max3:

mflr r0 ; save LR to r0stw r0,4(rsp) ; then to stack

stwu rsp,-16(rsp) ; create frame

stw r31,12(rsp) ; save r31

; r31 will be used to hold z ; function body

; epilogue

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Function Prologue/EpilogueEpilogue for max3

max3:

; prologue; function body

lwz r31,12(rsp) ; restore r31

addi rsp,rsp,16 ; release frame

lwz r0,4(rsp) ; get old LR valuemtlr r0 ; move to LR

blr ; return

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Function Prologue/Epiloguemax3:mflr r0 ; move LR to r0stw r0,4(rsp) ; save to current framestwu rsp,-16(rsp) ; create a new framestw r31,12(rsp) ; save r31mr r31,r5 ; put z into 31 (nonvolatile)

bl max ; call maxmr r4,r31 ; put z into r4 (second parameter)bl max ; call maxlwz r31,12(rsp) ; restore old r31addi rsp,rsp,16 ; release the frame

lwz r0,4(rsp) ; get the old LR valuemtlr r0 ; move back to LRblr ; return

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Beyond AssemblyAssembly programming => understanding of machine-level execution

Future uses Mixed C/assembly programming for embedded systems Computer organization and architecture

Compiler code generation and optimization

Operating Systems

Security

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Function calls.ppt