Assembly 07. Outline Boxes within Boxes Procedure Definition call, ret Saving / Restoring Registers Argument(s) Return Value(s) Global vs. Local Data.

Assembly 07

2

Outline

• Boxes within Boxes• Procedure Definition• call, ret• Saving / Restoring Registers• Argument(s)• Return Value(s)• Global vs. Local Data• Accidental Recursion

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Boxes within Boxes

• Procedures help manage code complexity• Procedures make code more:

• Readable• Maintainable• Reusable

What does this code do

again?

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Boxes within Boxes

• Unlike high-level languages, assembly does not “ship” with built-in procedures (e.g., printf in C++)

• You have to write your own procedures• Read from file• Write to file• Etc.

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Boxes within Boxes

• Book uses example of “getting up in the morning”• Shut off clock radio• Climb out of bed• Let dogs out• Eat breakfast• Brush your teeth• Shower

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Boxes within Boxes

• Each task can be divided into smaller tasks:• E.g., Brushing your teeth:

• Pick up toothpaste• Unscrew cap• Place cap on sink counter• …

• Same idea with procedures• Divide tasks into subtasks

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Outline

• Boxes within Boxes• Procedure Definition• call, ret• Saving / Restoring Registers• Argument(s)• Return Value(s)• Global vs. Local Data• Accidental Recursion

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Procedure Definition

• Must begin with a label• Must have at least one ret (return)• E.g.,

my_print: ; label for “my_print” proc<instruction> ; some instruction<instruction> ; some instructionret ; return

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Outline

• Boxes within Boxes• Procedure Definition• call, ret• Saving / Restoring Registers• Argument(s)• Return Value(s)• Global vs. Local Data• Accidental Recursion

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call Mnemonic

call -> used to invoke the procedure

usage: call <procedure_name>;

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ret Mnemonic

ret -> returns from procedure

usage:ret ; takes no operands

; returns to instruction after call

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call / ret Mnemonics

• call puts esp onto stack• esp has address of next instruction (after call)

• ret pops esp from stack• Execution resumes at the instruction after call

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack

esp

1. instruction executes

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack

esp

2. call to my_print made

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack

esp

3. esp+1 value pushed to stack

address of <next instr>;

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack

esp

4. flow of execution goes to my_print

address of <next instr>;

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack5. my_print executes address of <next instr>;

esp

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack6. ret (return) reached in my_print

address of <next instr>;

esp

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack7. Address popped off stack

8. esp set to address

address of <next instr>;

esp

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack9. Flow of execution continues at instruction after call

esp

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my_print:

<instruction>;

<instruction>;

ret;

<instruction>;

call my_print;

<next instr>;

…

stack10. Flow of execution continues…

esp

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stack

Every time you make a call, the stack grows by 32 bits.

Why??

return address

return address

return address

return address

return address

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stack

Every time you return from a procedure (with ret), the stack shrinks by 32 bits.

return address

return address

return address

msg: db “Hello There!!!”,10 ; in .datamsgLen: equ $-msg ; in .data

call my_print ; in .textcall my_print ; in .textcall my_print ; in .text

my_print: ; in .textmov eax, 4 ; make sys_write callmov ebx, 1 ; write to stdoutmov ecx, msg ; write contents of msgmov edx, msgLen ; number of bytes to write int 0x80 ; make system callret ; return

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call / ret Mnemonics

UNIX> ./a.outHello World!!!Hello World!!!Hello World!!!UNIX>

msg: db “Hello There!!!”,10msgLen: equ $-msg call my_printcall my_printcall my_print

my_print:mov eax, 4mov ebx, 1mov ecx, msgmov edx, msgLenint 0x80ret

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Outline

• Boxes within Boxes• Procedure Definition• call, ret• Saving / Restoring Registers• Argument(s)• Return Value(s)• Global vs. Local Data• Accidental Recursion

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Saving Register Values

• What is the result of add eax, ebx?

mov eax, 42;mov ebx, 58;call my_print;add eax, ebx;

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Saving Register Values

mov eax, 42;mov ebx, 58;call my_print;add eax, ebx;

eax

ebx

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Saving Register Values

mov eax, 42;mov ebx, 58;call my_print;add eax, ebx;

42

58

eax

ebx

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Saving Register Values

mov eax, 42;mov ebx, 58;call my_print;add eax, ebx;

4

1

eax

ebx

my_print sets eax to “4” for sys_write

my_print sets ebx to “1” for stdout

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Saving Register Values

mov eax, 42;mov ebx, 58;call my_print;add eax, ebx;

5

1

eax

ebx

were we expecting “5” or “100”??

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Saving Register Values

• Often important to save registers before calling a procedure

• Guards against bugs that are extremely hard to track down

spooky

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Saving Register Values

• Use the stack to save registers• Push registers onto stack before procedure call• Pop registers off of stack after procedure returns

• Be mindful of order!• Last In First Out

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Saving Register Values

• If you “own” the procedure code, you can push / pop registers within the procedure

• Push registers at beginning of procedure

• Pop registers at end of procedure

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Saving Register Values

mov eax, 42;mov ebx, 58;pushad;call my_print;popad;add eax, ebx;

eax

ebx

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Saving Register Values

mov eax, 42;mov ebx, 58;pushad;call my_print;popad;add eax, ebx;

42

58

eax

ebx

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Saving Register Values

mov eax, 42;mov ebx, 58;pushad;call my_print;popad;add eax, ebx;

42

58

eax

ebx

all 32-bit GP registers pushed to stack

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Saving Register Values

mov eax, 42;mov ebx, 58;pushad;call my_print;popad;add eax, ebx;

4

1

eax

ebx

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Saving Register Values

mov eax, 42;mov ebx, 58;pushad;call my_print;popad;add eax, ebx;

42

58

eax

ebx

pop all 32-bit registers from stack back to registers

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Saving Register Values

mov eax, 42;mov ebx, 58;pushad;call my_print;popad;add eax, ebx;

100

58

eax

ebx

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Outline

• Boxes within Boxes• Procedure Definition• call, ret• Saving / Restoring Registers• Argument(s)• Return Value(s)• Global vs. Local Data• Accidental Recursion

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Passing Arguments

• How do you pass arguments to assembly procedures?• … and no, this is not a setup for a hilarious joke…

• Use registers!• Caller puts arguments into pre-ordained registers• E.g., eax is argument 1, ebx is argument 2, etc.

• Important to clearly comment your procedures!!• Especially expectations for arguments• E.g., integer in eax, pointer in ebx, etc.

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Passing Arguments

• Example: procedure to add two numbers, print result to stdout• Numbers will be arguments to procedure

• Simplified version: only prints results between 0 and 9…• Printing results > 10 may be part of your homework…

SIZE: equ 10 ; in .dataoutput: resb SIZE ; in .bss

; below in .textmov eax, 2 ; argument 1 in eaxmov ebx, 3 ; argument 2 in ebxcall my_add ; invoke procedure to add/print

my_add: ; procedure to add / print resultadd eax, ebx ; add argumentsadd eax, ‘0’ ; convert to ASCII numbermov [output], eax; ; put result in output buffermov byte [output+1], 10 ; add carriage returncall my_print ; write output buffer to stdoutret ; return to caller

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Procedure Arguments

UNIX> ./a.out5UNIX>

msg: SIZE: equ 10output: resb SIZE

mov eax, 2mov ebx, 3call my_add

my_add:add eax, ebxadd eax, ‘0’mov [output], eaxmov byte [output+1], 10 call my_printret

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Outline

• Boxes within Boxes• Procedure Definition• call, ret• Saving / Restoring Registers• Argument(s)• Return Value(s)• Global vs. Local Data• Accidental Recursion

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Return Value(s)

• How do you return value(s) from assembly procedures?

• Register(s), of course!!

• Example: convert character from lowercase to uppercase

mov al, ‘a’ ; argument 1 in alcall convert ; convert ‘a’ to ‘A’

; cl now has converted

covert: ; procedure to convertmov cl, al ; copy argument 1sub cl, 32 ; make return value uppercase

; (‘A’ is 32 less than ‘a’)ret ; return to caller

mov al, ‘a’;call convert;

covert:mov cl, alsub cl, 32ret

al

cl

mov al, ‘a’;call convert;

covert:mov cl, alsub cl, 32ret

96 = 0x61 = ‘a’al

cl

mov al, ‘a’;call convert;

covert:mov cl, alsub cl, 32ret

96 = 0x61 = ‘a’al

cl

mov al, ‘a’;call convert;

covert:mov cl, alsub cl, 32ret

96 = 0x61 = ‘a’

96 = 0x61 = ‘a’

al

cl

mov al, ‘a’;call convert;

covert:mov cl, alsub cl, 32ret

96 = 0x61 = ‘a’

65 = 0x41 = ‘A’

al

cl

mov al, ‘a’;call convert;

covert:mov cl, alsub cl, 32ret

96 = 0x61 = ‘a’

65 = 0x41 = ‘A’

al

cl

now cl has return value ‘A’

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Outline

• Boxes within Boxes• Procedure Definition• call, ret• Saving / Restoring Registers• Argument(s)• Return Value(s)• Global vs. Local Data• Accidental Recursion

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Global vs. Local Data

• Global: can be accessed anywhere in code• Local: can only be accessed “locally” (e.g., within procedure)

• In x86 assembly, all declared data items are global• Can declare data items in .text (!!!)

• Appear local, but actually global

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Examples of Local Data

• Using the stack to temporarily store data in procedures• Data pushed to stack (during procedure) only visible to that procedure

• Data items declared in external code libraries• Only visible to external code • (But there is a mechanism to make global)

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Outline

• Boxes within Boxes• Procedure Definition• call, ret• Saving / Restoring Registers• Argument(s)• Return Value(s)• Global vs. Local Data• Accidental Recursion

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Accidental Recursion

• “Uncommon but inevitable bug”

• Watch out for incorrect base case• Pay attention to instruction => flags

• Will eventually run out of stack space• Each call pushes 32-bits onto stack (Why?)• Eventually causes a Segmentation Fault (Why?)

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Accidental Recursion Example

• “Not so accidental” recursion

call recur; ; 1st instruction in .text ; after _start

recur:call my_print ; call procedure to print “LINE”call recur ; make recursive callret ; never reached..

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Accidental Recursion Example

UNIX> ./a.outLINELINELINELINELINE... (on and on..)Segmentation FaultUNIX>

call recur;

recur:call my_printcall recurret

stdout in BLUEstderr in GREEN

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Accidental Recursion Example

UNIX> ./a.out > output.txtSegmentation FaultUNIX> head –n 2 output.txtLINELINEUNIX> wc –l output.txt 2094543 output.txt

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Accidental Recursion Example

UNIX> ./a.out > output.txtSegmentation FaultUNIX> head –n 2 output.txtLINELINEUNIX> wc –l output.txt 2094543 output.txt

> : redirect stdout to a file named “output.txt”

stderr ignored

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Accidental Recursion Example

UNIX> ./a.out > output.txtSegmentation FaultUNIX> head –n 2 output.txtLINELINEUNIX> wc –l output.txt 2094543 output.txt

head –n 2 output.txt : output top 2 lines of file output.txtoutput.txt contains “LINE” printed once per line

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Accidental Recursion Example

UNIX> ./a.out > output.txtSegmentation FaultUNIX> head –n 2 output.txtLINELINEUNIX> wc –l output.txt 2094543 output.txt

wc –l output.txt :count number of lines in file output.txt

output.txt contains 2,094,543 lines.recur called 2+ million times!!