Lessons on byte alignment in learning about ARM undefined instruction exceptions-EEWORLD

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Today, I learned the course of undefined instruction exception of jz2440 development board. After following teacher Wei Dongshan to write code, compile and burn, the program teacher could not run normally. After careful investigation, it was found that the byte was not aligned to 4 bytes. The start.S file in the previous program started as follows:

_start:

/* Disable watchdog */

ldr r0, =0x53000000

ldr r1, =0

str r1, [r0]

The instructions are aligned to 4 bytes from the beginning, so there is no problem running the program. But when the undefined instruction exception handling statement is added after the _start label, the code is as follows:

_start:

b reset

b do_and

do_and:

ldr sp, =0x34000000

stmdb sp!, {r0-r12, lr}

mrs r0, cpsr

ldr r1, =and_string

bl printException

ldmia sp!, {r0-r12, pc}^

and_string:

.string "underdefined instruction exception"

.align 4

reset:

/* Disable watchdog */

ldr r0, =0x53000000

ldr r1, =0

str r1, [r0]

If you do not add the .align 4 statement before the reset label, the address of the compiled reset segment code will be 0x300000043, which is not 4-byte aligned, so the program cannot jump and execute normally after startup.

Keywords：ARM Reference address：Lessons on byte alignment in learning about ARM undefined instruction exceptions

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