;
;	CPE 221 Assembly Example
;	3/6/18
;	This program multiplies two numbers by repeated addition.
;	First, take absolute values of both numbers, do multiplication,
; 	then adjust result as necessary.

        AREA MULTIPLY_BY_ADDING, CODE, READONLY
        ENTRY
        LDR  r0, num1		     ; Put num1 in r0.
        LDR  r1, num2		     ; Put num2 in r1.
        MOV  fp, #0x0000C000 
        MOV  sp, #0x00000000        
        ADR  r10, case1
mpy_ne  MOV   r3, #0		        ; Set r3 to 0, it will hold the result.
        TEQ   r0, #0                ; Compare first parameter to 0
        BEQ   done                  ; If first parameter is 0 we are done, result = 0.
        TEQ   r1, #0                ; Compare second parameter to 0
        BEQ   done                  ; If second parameter is 0, we are done, result = 0.
        PUSH  {fp}                 ; Save old frame pointer in preparation for branching
        MOV   fp, sp               ; Copy stack pointer into the frame pointer
        SUB   sp, sp, #8            ; Make space on the stack for one input, one output
        STR   r1, [fp, #-4]        ; Store the input parameter on the stack
        BL    abs                   ; Branch to abs routine and store return in lr
        LDR   r4, [fp, #-8]        ; Copy result from the stack into r4
        SUB   sp, sp, #8            ; Make space on the stack for one input, one output
        STR   r0, [fp, #-4]        ; Store the input parameter on the stack
        BL    abs                   ; Branch to abs routine and store return in lr
        LDR   r5, [fp, #-8]        ; Copy result from the stack into r5
        MOV   sp, fp               ; Collapse the frame by moving the stack pointer
        POP   {fp}                 ; Pull the old frame pointer off the stack      
adding  ADD   r3, r3, r5		    ; Add num2.
        SUBS  r4, r4, #1		    ; Decrement r4, the abs of num1.
        BEQ   adjust		        ; If r4 = 0, done adding, go to adjust.
        B     adding		        ; Otherwise, need to add again.
adjust  MOVS  r0, r0		        ; Done adding, now adjust sign of result.
        RSBMI r3, r3, #0		    ; If num2 negative, negate result.
        MOVS  r1, r1                   
        RSBMI r3, r3, #0 		    ; If num1 negative, negate result.
done    STR  r3, [r10]           ; Store the result
final   B     final 
abs     LDR   r9, [fp, #-4]        ; Copy the input parameter off of the stack
        CMP   r9, #0                ; Test input parameter 
        BPL   d_abs                 ; If zero or greater, we're done
        RSB   r9, r9, #0            ; If negative, make it positive
d_abs   STR   r9, [fp, #-8]        ; Store the result on the stack
        MOV   pc, lr                  ; Pull the saved lr value off stack and put in pc to return
num1    DCD   -3                     ; Give num1 a value
num2    DCD   2                     ; Give num2 a value
case1   SPACE 4                     ; Make space for result
        END