Function Calls and EABI

1
Function Calls and EABI

• Assembly Program Layout
• Function Call Overview
• Parameter and Result Passing
• Function Variables

2
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

3
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

directive for text (code) segment
label
4
Hello World in Assembly

• bl LCD_Init call LCD_Init
• lis r3,mystring_at_ha load mystring upper half
• addi r3,r3,mystring_at_l load mystring lower
  half
• bl LCD_PutString call LCD_PutString
• li r3,0 clear r3
• addi rsp,rsp,8 restore stack pointer
• lwz r0,4(rsp) load original LR value into r0
• mtlr r0 move r0 value into LR
• blr end of main
• .data
• mystring .ascii "Hello World!\r"

directive for data segment
label
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Passing Parameters and Results
EABI Register Usage ConventionsEABI Embedded
Application Binary Interface
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Passing Parameters and Result

• int max(int x, int y)
• int max
• if (x gt y)
• max x
• else
• max y
• return max
• int max3(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_greater
• b 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

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 later
• stw r0,4(rsp)
• stwu rsp,-16(rsp)
• stw r31,12(rsp)
• mr r31,r5 save z to r31 r31 is
  nonvolatile
• bl max call max
• mr r4,r31 now r3max(x,y), r4z
• bl max call max
• lwz 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 Calls

• max3
• mflr r0 save LR to r0
• stw r0,4(rsp) now LR saved into stack
• stwu rsp,-16(rsp) create 16 bytes in stack
  AND save old SP
• stw r31,12(rsp) save nonvolatile r31 into
  stack
• mr r31,r5
• bl max call max
• mr r4,r31
• bl max call max
• lwz r31,12(rsp) restore r31 value
• addi rsp,rsp,16 release 16 byte from stack
  AND restore SP
• lwz r0,4(rsp) restore old LR value
• mtlr r0 move to LR
• blr return

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Nested Function Calls
EABI stack usage

• int max(int x, int y)
• Which areas are saved in stack?
• int max3(int x, int y, int z)
• Which areas are saved in stack?
• Exercise Give the layout of max3 stack usage

High-end address
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
Load-end address
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Function Exercises
add_array.c

• void add_array (int X, int Y, int N)
• int i
• for (i 0 i lt N i)
• Xi Yi

Write add_array.asm to replace this program
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Beyond Assembly

• Assembly programming gt 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