@ Use these commands to assemble, link, run and debug this program:
@ as -o student_inputC.o student_inputC.s
@ gcc -o student_inputC student_inputC.o
@ ./student_inputC
@ gdb --args ./student_inputC
.equ READERROR, 0 @Used to check for scanf read error.
.global main @ Have to use main because of C library uses.
main:
prompt:
@ Ask the user to enter a number.
ldr r0, =strInputPrompt
@ Put the address of my string into the first parameter
bl printf
@ Call the C printf to display input prompt.
@ Ask the user to enter a character.
ldr r0, =charInputPrompt
bl printf
get_input:
@ Set up r0 with the address of input pattern.
@ scanf puts the input value at the address stored in r1. We are going
@ to use the address for our declared variable in the data section - intInput.
@ After the call to scanf the input is at the address pointed to by r1 which
@ in this case will be intInput.
ldr r0, =numInputPattern @ Setup to read in one number.
ldr r1, =intInput @ load r1 with the address of where the
@ input value will be stored.
bl scanf @ scan the keyboard.
cmp r0, #READERROR @ Check for a read error.
beq readerror @ If there was a read error go handle it.
ldr r1, =intInput @ Have to reload r1 because it gets wiped out.
ldr r1, [r1] @ Read the contents of intInput and store in r1 so that
ldr r0, =charInputPattern @ Setup to read in one number.
ldr r1, =charInput @ load r1 with the address of where the
bl scanf @ scan the keyboard.
cmp r0, #READERROR @ Check for a read error.
beq readerror @ If there was a read error go handle it.
ldr r1, =charInput @ Have to reload r1 because it gets wiped out.
ldr r1, [r1] @ Read the contents of intInput and store in r1 so that it
@ Print the input out as a number.
@ r1 contains the value input to keyboard.
ldr r0, =strOutputNum
bl printf
b myexit
ldr r0, =strOutputChar
bl printf
b myexit @ leave the code.
readerror:
@ Got a read error from the scanf routine. Clear out the input buffer then
@ branch back for the user to enter a value.
@ Since an invalid entry was made we now have to clear out the input buffer by
@ reading with this format %[^n] which will read the buffer until the user
@ presses the CR.
ldr r0, =strInputPattern
ldr r1, =strInputError @ Put address into r1 for read.
bl scanf @ scan the keyboard.
@ Not going to do anything with the input. This just cleans up the input buffer.
@ The input buffer should now be clear so get another input.
b prompt
myexit:
@ End of my code. Force the exit and return control to OS
mov r7, #0x01 @ SVC call to exit
svc 0 @ Make the system call.
.data
@ Declare the strings and data needed
.balign 4
strInputPrompt: .asciz "Input the number: n"
.balign 4
strOutputNum: .asciz "The number value is: %d n"
.balign 4
charInputPrompt: .asciz "Input the character: n"
.balign 4
strOutputChar: .asciz "The character is: %c n"
@ Format pattern for scanf call.
.balign 4
numInputPattern: .asciz "%d" @ integer format for read.
.balign 4
strInputPattern: .asciz "%[^n]" @ Used to clear the input buffer for invalid input.
.balign 4
strInputError: .skip 100*4 @ User to clear the input buffer for invalid input.
.balign 4
intInput: .word 0 @ Location used to store the user input.
.balign 4
charInputPattern: .asciz "%c" @ integer format for read.
.balign 4
charInput: .word 0 @ Location used to store the user input
@ Let the assembler know these are the C library functions.
.global printf
@ To use printf:
@ r0 - Contains the starting address of the string to be printed. The string
@ must conform to the C coding standards.
@ r1 - If the string contains an output parameter i.e., %d, %c, etc. register
@ r1 must contain the value to be printed.
@ When the call returns registers: r0, r1, r2, r3 and r12 are changed.
.global scanf
@ To use scanf:
@ r0 - Contains the address of the input format string used to read the user
@ input value. In this example it is numInputPattern.
@ r1 - Must contain the address where the input value is going to be stored.
@ In this example memory location intInput declared in the .data section
@ is being used.
@ When the call returns registers: r0, r1, r2, r3 and r12 are changed.
@ Important Notes about scanf:
@ If the user entered an input that does NOT conform to the input pattern,
@ then register r0 will contain a 0. If it is a valid format
@ then r0 will contain a 1. The input buffer will NOT be cleared of the invalid
@ input so that needs to be cleared out before attempting anything else.
@
@ Additional notes about scanf and the input patterns:
@ 1. If the pattern is %s or %c it is not possible for the user input to generate
@ and error code. Anything that can be typed by the user on the keyboard
@ will be accepted by these two input patterns.
@ 2. If the pattern is %d and the user input 12.123 scanf will accept the 12 as
@ valid input and leave the .123 in the input buffer.
@ 3. If the pattern is "%c" any white space characters are left in the input
@ buffer. In most cases user entered carrage return remains in the input buffer
@ and if you do another scanf with "%c" the carrage return will be returned.
@ To ignore these "white" characters use " $c" as the input pattern. This will
@ ignore any of these non-printing characters the user may have entered.
@
@ End of code and end of file. Leave a blank line after this.
发生这种情况的原因非常简单。让我们仔细看一下代码:
LDR r0,=StringInputPrompt
BL printf
LDR r0,=charInputPrompt
BL printf
现在忽略分支预测符和乱序执行，汇编按照编写的顺序执行，从上到下。你正在加载一个字符串上面写着输入数字输入R0，然后打印出来，然后输入字符串"然后打印出来。您已经打印了请求输入的字符串，但在打印完两个提示符之前没有接受任何实际输入。
接下来，我们将查看getInput部分:
get_input:
ldr r0, =numInputPattern @ Setup to read in one number.
ldr r1, =intInput @ load r1 with the address of where the
bl scanf @ scan the keyboard
cmp r0, #READERROR @ Check for a read error.
beq readerror @ If there was a read error go handle it.
ldr r1, =intInput @ Have to reload r1 because it gets wiped out.
ldr r1, [r1] @
ldr r0, =charInputPattern @ Setup to read in one number.
ldr r1, =charInput @ The problem is here! (added by me)
bl scanf @ scan the keyboard.
cmp r0, #READERROR @ Check for a read error.
beq readerror @ If there was a read error go handle it.
ldr r1, =charInput @ Have to reload r1 because it gets wiped out.
ldr r1, [r1]
再一次，您正确地接受了输入，但是随后您用=charInput覆盖了r1。了解汇编的重要一点是寄存器和永久内存之间的区别。看看这个:
LDR R0,=intInput
LDR R0,=charInput
有这两个背靠背的就等于没有第一个，即
LDR R0,=charInput
因为每个寄存器一次只能包含一个值，当你加载一个新值时，旧的就会被丢弃。
你的代码的其余部分似乎是工作的，所以一个简单的重新安排你的代码应该修复它:
main:
@ Ask the user to enter a number.
ldr r0, =strInputPrompt
@ Put the address of my string into the first parameter
bl printf
@ Set up r0 with the address of input pattern.
ldr r0, =numInputPattern @ Setup to read in one number.
ldr r1, =intInput @ load r1 with the address of where the
bl scanf @ scan the keyboard.
cmp r0, #READERROR @ Check for a read error.
beq readerror @ If there was a read error go handle it.
ldr r1, =intInput @ Have to reload r1 because it gets wiped out.
ldr r1, [r1]
@ Print the input out as a number.
ldr r0, =strOutputNum
bl printf
@ Ask the user to enter a character.
ldr r0, =charInputPrompt
bl printf
ldr r0, =charInputPattern @ Setup to read in one number.
ldr r1, =charInput @ load r1 with the address of where the
bl scanf @ scan the keyboard.
cmp r0, #READERROR @ Check for a read error.
beq readerror @ If there was a read error go handle it.
ldr r1, =charInput @ Have to reload r1 because it gets wiped out.
ldr r1, [r1]
ldr r0, =strOutputChar
bl printf
b myexit @ leave the code.
TL;博士:汇编中指令的顺序很重要。很多。