ARM汇编语言菜单不工作

Question

/*menu_prompt*/ 
.balign 4 
menu_prompt: .asciz "Choose which problem.\nType 1 for Problem 1\nType 2 for Pr$ 
/*scanner*/ 
.balign 4 
scan_pattern_int: .asciz "%d" 
scan_read: .word 0 
.balign 4 
return: .word 0 


.global main 
main: 
    ldr r0, address_of_return 
    str lr, [r1] 

    ldr r0, address_of_menu_prompt 
    bl printf 

    ldr r0, address_of_scan_pattern_int 
    ldr r1, address_of_scan_read 
    bl scanf 
    ldr r1, [r1] 
    str lr, [r1] 

    ldr r0, address_of_menu_prompt 
    bl printf 

    ldr r0, address_of_scan_pattern_int 
    ldr r1, address_of_scan_read 
    bl scanf 
    ldr r1, [r1] 
    cmp r1, #1 
    beq in_prob_1 
    cmp r1, #2 
    beq in_prob_2 
    cmp r1, #3 
    beq in_prob_3 
in_prob_1: 
    mov r0, #100 
    bal end 
in_prob_2: 
    mov r0, #200 
    bal end 
in_prob_3: 
    mov r0, #300 
    bal end 
end: 
    ldr lr, address_of_return 
    ldr lr, [lr]      
    bx lr 

address_of_menu_prompt: .word menu_prompt 
address_of_scan_pattern_int: .word scan_pattern_int 
address_of_scan_read: .word scan_read 
address_of_return: .word return 

运行程序后，无论我选择哪种输入（1,2或3）echo $？总是表明r0拥有139，我不知道为什么。如果我选择1 r0应该保持100，2 r0应该保持200,3和r0应该保持300.

Answer 1

scanf不需要保留r1的内容，因为您已经通过ARM过程调用标准传递。分支到scanf后，您应该重新加载r1。