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@ Define my Raspberry Pi -- modified for the Pi Zero
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        .cpu    arm1176jzf-s 
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        .fpu    vfp
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        .syntax unified         @ modern syntax
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@ Useful source code constants
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        .equ    STDIN,      0
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        .equ    STDOUT,     1
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        .equ    aLetter,    -8      @ as discussed in class
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        .equ    local,      8
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@ Constant program data
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        .section    .rodata
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        .align      2
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promptMsg:
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        .asciz      "Enter a character: "
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        .equ        promptLngth,.-promptMsg
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responseMsg:
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        .asciz      "You entered: "
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        .equ        responseLngth,.-responseMsg
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@ Program code
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        .text
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        .align      2
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        .global     main
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        .type       main, %function
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main:
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        sub     sp, sp, 8               @ space for fp, lr
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        str     fp, [sp, 0]             @ save fp
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        str     lr, [sp, 4]     	@   and lr
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        add     fp, sp, 4       	@ set our frame pointer
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        sub     sp, sp, local   	@ allocate memory for local var
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        mov     r0, STDOUT      	@ prompt user for input
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        ldr     r1, promptMsgAddr
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        mov     r2, promptLngth
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        bl      write
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        mov     r4, aLetter     	@ address to write to -- THIS IS A HINT
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        mov     r0, STDIN       	@ from keyboard
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        add     r1, fp, r4      	@ address of aLetter[0] -- THIS IS ALSO A HINT
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        mov     r2, 4            	@ one char is changed to four chars
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        bl      read
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        mov     r0, STDOUT      	@ nice message for user
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        ldr     r1, responseMsgAddr
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        mov     r2, responseLngth
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        bl      write
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        mov     r0, STDOUT      	@ echo user's character
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        add     r1, fp, aLetter 	@ address of aLetter
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        mov     r2, 4           	@ one char is changed to four chars
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        bl      write
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        mov     r0, 0           	@ return 0;
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        add     sp, sp, local   	@ deallocate local var
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        ldr     fp, [sp, 0]     	@ restore caller fp
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        ldr     lr, [sp, 4]     	@       lr
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        add     sp, sp, 8       	@   and sp
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        bx      lr              	@ return
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@ Addresses of messages
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        .align  2
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promptMsgAddr:
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        .word   promptMsg
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responseMsgAddr:
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        .word   responseMsg
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