Exercises 14.2 Programming Exercise
1.
Write a program in assembly language that (a) prompts the user to enter some alphabetic characters, (b) converts any uppercase characters to lowercase, and (c) displays the resulting uppercase string. Your algorithm must allow for the user to enter lowercase characters and leave them as lowercase.
Hint
Solution
Consult Table 2.13.1 to see which bit in the ASCII code determines upper/lower case.
@ lowerCase.s
@ Prompts user to enter alphabetic characters, converts
@ all uppercase to lowercase and shows the result.
@ 2017-09-29: Bob Plantz
@ Define my Raspberry Pi
.cpu cortex-a53
.fpu neon-fp-armv8
.syntax unified @ modern syntax
@ Constant for assembler
.equ nBytes,50 @ amount of memory for string
@ Constant program data
.section .rodata
.align 2
prompt:
.asciz "Enter some alphabetic characters: "
@ The program
.text
.align 2
.global main
.type main, %function
main:
sub sp, sp, 16 @ space for saving regs
@ (keeping 8-byte sp align)
str r4, [sp, 4] @ save r4
str fp, [sp, 8] @ fp
str lr, [sp, 12] @ lr
add fp, sp, 12 @ set our frame pointer
mov r0, nBytes @ get memory from heap
bl malloc
mov r4, r0 @ pointer to new memory
ldr r0, promptAddr @ prompt user
bl writeStr
mov r0, r4 @ get user input
mov r1, nBytes @ limit input size
bl readLn
mov r0, r4 @ convert to lowercase
bl toLower
mov r0, r4 @ echo user input
bl writeStr
mov r0, r4 @ free heap memory
bl free
mov r0, 0 @ return 0;
ldr r4, [sp, 4] @ restore r4
ldr fp, [sp, 8] @ fp
ldr lr, [sp, 12] @ lr
add sp, sp, 16 @ restore sp
bx lr @ return
promptAddr:
.word prompt
@ toLower.s
@ Converts all alpha characters to lowercase.
@ Calling sequence:
@ r0 <- address of string to be written
@ bl toLower
@ returns number of characters written
@ 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 lowerMask,0x20
.equ NUL,0
@ The code
.text
.align 2
.global toLower
.type toLower, %function
toLower:
sub sp, sp, 16 @ space for saving regs
str r4, [sp, 0] @ save r4
str r5, [sp, 4] @ r5
str fp, [sp, 8] @ fp
str lr, [sp, 12] @ lr
add fp, sp, 12 @ set our frame pointer
mov r4, r0 @ r4 = string pointer
mov r5, #0 @ r5 = count
whileLoop:
ldrb r3, [r4] @ get a char
cmp r3, NUL @ end of string?
beq allDone @ yes, all done
orr r3, r3, lowerMask @ convert to lowercase
strb r3, [r4] @ update string
add r4, r4, 1 @ increment pointer var
add r5, r5, 1 @ count++
b whileLoop @ back to top
allDone:
mov r0, r5 @ return count;
ldr r4, [sp, 0] @ restore r4
ldr r5, [sp, 4] @ r5
ldr fp, [sp, 8] @ fp
ldr lr, [sp, 12] @ lr
add sp, sp, 16 @ restore sp
bx lr @ return