Functions in ARM Assembly – Nested Function

Calls

Chris McCarthy

FUNCTIONS IN ASM

• Not 'native' to assembly
• We need to do a lot of the management ourselves
• Argument passing:
• How do we pass arguments from one function to another
• Storing and recalling register values
• each function we call will want to use the same registers (only 13 general purpose registers!)
• How do we manage this?
• Managing the program control
• Jumping from one function to another, and then returning back!

• Not 'native' to assembly
• We need to do a lot of the management ourselves
• Argument passing:
• How do we pass arguments from one function to another
• Storing and recalling register values
• each function we call will want to use the same registers (only 13 general purpose registers!)
• How do we manage this?
• Managing the program control
• Jumping from one function to another, and then returning back!

RECALL OUR “FLASHING LED” PROGRAM

FROM LAST LECTURE

RECALL OUR “FLASHING LED” PROGRAM

FROM LAST LECTURE

• Last lecture we created a function to call for delays.

RECALL OUR “FLASHING LED” PROGRAM

FROM LAST LECTURE

• But we can decompose this further!
• Think about the flash code

RECALL OUR “FLASHING LED” PROGRAM

FROM LAST LECTURE

• But we can decompose this further!
• Let’s write a flash function, and pass it a colour to flash, and a time delay between flashes.

RECALL OUR “FLASHING LED” PROGRAM

FROM LAST LECTURE

• But we can decompose this further!
• The flash code still has some repetitive code.

RECALL OUR “FLASHING LED” PROGRAM

FROM LAST LECTURE

• The only difference between these blocks is the colour of the pixel being drawn.
• We can write this as a function and pass the colour (and delay time) as parameters

DRAW PIXEL FUNCTION

drawpixel:

PUSH {R3,R4}

MOV R3, R0 ; copy pixel colour to R3
MOV R4, R1 ; copy delay time to R4
STR R3, .Pixel367 ; draw pixel with colour
PUSH {R0}
MOV R0, R4 ; pass delay time to delay function

BL delay; call delay function

Pop {R0}

RET; job done

FLASH LOOP (NOW CALLING DRAWPIXEL)

mov R2, #1 ; 1sec delay time

flash:

MOV R0, #.green ; pass green to drawpixel
MOV R1, R2 ; pass time delay

BL drawpixel; "flash on“

MOV R0, #.white ; pass white to drawpixel
MOV R1, R2 ; pass time delay

BL drawpixel; "flash off" B flash

HALT

AWESOME – BUT DOES IT WORK?

• Let’s see it in ARMlite
• Spoiler alert … no … but can you see why?
• Rewind video and take another look at drawpixel
• Hint: think about what happens to LR when functions call other functions

DRAW PIXEL FUNCTION (FIXED)

drawpixel:

PUSH {R3,R4}

MOV R3, R0 ; copy pixel colour to R3
MOV R4, R1 ; copy delay time to R4
STR R3, .Pixel367 ; draw pixel with colour
PUSH {R0}
MOV R0, R4 ; pass delay time to delay function

PUSH {LR}; backup LR before BL delay overwrites it! BL delay; call delay function POP {LR}; restore LR after we’ve returned from delay

Pop {R0}

RET; job done

SUMMARY

• All of this is interesting because this is exactly what compilers have to handle when translating your code in higher level languages
• Critical components include LR and PC registers,
• But also the stack!
• The stack allows functions to do what they do using all registers, and then restores what was there previously
• This is also critical to interrupt handling
• we’ll come back to this