Reverse Engineering for Beginners

CHAPTER 14. LOOPS CHAPTER 14. LOOPS

CMP R4, #0xA

BLT loc_35C

MOV R0, #0

LDMFD SP!, {R4,PC}

Iteration counteriis to be stored in theR4register.

The“MOV R4, #2”instruction just initializesi.

The“MOV R0, R4”and“BL printing_function”instructions compose the body of the loop, the first instruction

preparing the argument forf()function and the second calling the function.

The“ADD R4, R4, #1”instruction just adds 1 to theivariable at each iteration.

“CMP R4, #0xA”comparesiwith0xA(10). The next instructionBLT(Branch Less Than) jumps ifiis less than 10.

Otherwise, 0 is to be written intoR0(since our function returns 0) and function execution finishes.

Optimizing Keil 6/2013 (Thumb mode)

_main

PUSH {R4,LR}

MOVS R4, #2

loc_132 ; CODE XREF: _main+E

MOVS R0, R4

BL printing_function

ADDS R4, R4, #1

CMP R4, #0xA

BLT loc_132

MOVS R0, #0

POP {R4,PC}

Practically the same.

Optimizing Xcode 4.6.3 (LLVM) (Thumb-2 mode)

_main

PUSH {R4,R7,LR}

MOVW R4, #0x1124 ; "%d\n"

MOVS R1, #2

MOVT.W R4, #0

ADD R7, SP, #4

ADD R4, PC

MOV R0, R4

BLX _printf

MOV R0, R4

MOVS R1, #3

BLX _printf

MOV R0, R4

MOVS R1, #4

BLX _printf

MOV R0, R4

MOVS R1, #5

BLX _printf

MOV R0, R4

MOVS R1, #6

BLX _printf

MOV R0, R4

MOVS R1, #7

BLX _printf

MOV R0, R4

MOVS R1, #8

BLX _printf

MOV R0, R4

MOVS R1, #9

BLX _printf

MOVS R0, #0

POP {R4,R7,PC}