# this subroutine bubble sorts vectors of words

#

# inputs

#   r0 - start of vector

#   r1 - number of elements to sort

#

# no outputs

#

# locals

#   r4 - current pointer

#   r5 - inner counter

#   r6 - keep_going flag

#   r7 - first element

#   r8 - second element

 

.equ datum_size,4

.globl _bubble

.text

_bubble:

        stmfd   sp!, {r4, r5, r6, r7, r8, lr}   @ save variables to stack

        cmp     r1, #1                          @ number of elements must be > 1

        ble     end_outer                       @ stop if nothing to do

 

        sub     r5, r1, #1                      @ need n-1 comparisons

        mov     r4, r0                          @ initialise current pointer

        mov     r6, #0                          @ this register set when we swap

 

loop_start:

        ldr     r7, [r4], #datum_size           @ load one element

        ldr     r8, [r4]                        @ and next one

        cmp     r7, r8                          @ compare them

        ble     no_swap                         @ branch if second greater

 

        mov     r6, #1                          @ set keep_going flag

        sub     r4, r4, #datum_size             @ reset pointer to first element

        swp     r8, r8, [r4]                    @ exchange value in r8 and address in r4

        str     r8, [r4, #datum_size]!          @ store new r8 to incremented address

no_swap:

        subs    r5, r5, #1                      @ decrement counter

        bne     loop_start                      @ and restart loop if more needed

 

end_inner:

        cmp     r6, #0                          @ check keep_going flag

        beq     end_outer                       @ and leave if not set

 

        mov     r6, #0                          @ clear keep_going flag

        mov     r4, r0                          @ reset pointer

        sub     r5, r1, #1                      @ reset counter

        b       loop_start                      @ start another iteration

 

end_outer:

        ldmfd   sp!, {r4, r5, r6, r7, r8, pc}   @ restore state from stack and leave subroutime

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