Exercises 10.6 Programming Exercise
1.
Write an assembly language program that allows the user to enter four characters and then echoes them. What happens if the user enters three characters? What happens if the user enters fewer than three characters?
Hint
Solution
Start with the program in Listing 10.5.2. Do not try to do this with a loop. Simply copy the read sequence. And make sure to study Figure 10.5.4 very carefully.
Experiment with entering different numbers of characters, and you will see that the read function reads the newline character (enter key) as one character. Try entering “abcdls” (without the quotes). Notice that any characters beyond the four requested remain in the input buffer and are read by the shell when the program exits.
@ echo4chars.s
@ Prompts user to enter 4 characters and echoes them
@ 2017-09-29: Bob Plantz
@ Define my Raspberry Pi
.cpu cortex-a53
.fpu neon-fp-armv8
.syntax unified @ modern syntax
@ Useful source code constants
.equ STDIN,0
.equ STDOUT,1
.equ char1,-8
.equ char2,-7
.equ char3,-6
.equ char4,-5
.equ nChars,4
.equ local,8
@ Constant program data
.section .rodata
.align 2
promptMsg:
.asciz "Enter four characters: "
.equ promptLngth,.-promptMsg
responseMsg:
.asciz "You entered: "
.equ responseLngth,.-responseMsg
@ Program code
.text
.align 2
.global main
.type main, %function
main:
sub sp, sp, 8 @ space for fp, lr
str fp, [sp, 0] @ save fp
str lr, [sp, 4] @ and lr
add fp, sp, 4 @ set our frame pointer
sub sp, sp, local @ allocate memory for local var
mov r0, STDOUT @ prompt user for input
ldr r1, promptMsgAddr
mov r2, promptLngth
bl write
mov r0, STDIN @ from keyboard
add r1, fp, char1 @ address of 1st char
mov r2, 1 @ one char
bl read
mov r0, STDIN @ from keyboard
add r1, fp, char2 @ address of 2nd char
mov r2, 1 @ one char
bl read
mov r0, STDIN @ from keyboard
add r1, fp, char3 @ address of 3rd char
mov r2, 1 @ one char
bl read
mov r0, STDIN @ from keyboard
add r1, fp, char4 @ address of 4th char
mov r2, 1 @ one char
bl read
mov r0, STDOUT @ nice message for user
ldr r1, responseMsgAddr
mov r2, responseLngth
bl write
mov r0, STDOUT @ echo user's characters
add r1, fp, char1 @ address of 1st char
mov r2, nChars @ all four characters
bl write
mov r0, 0 @ return 0;
add sp, sp, local @ delete local vars
ldr fp, [sp, 0] @ restore caller fp
ldr lr, [sp, 4] @ lr
add sp, sp, 8 @ and sp
bx lr @ return
@ Addresses of messages
.align 2
promptMsgAddr:
.word promptMsg
responseMsgAddr:
.word responseMsg