Add a new function to the sample program which compares the values of NUM1 and N

Question

Add a new function to the sample program which compares the values of NUM1 and NUM2 and stores the larger value into a new variable LARGER.

a. Define a new 32bit variable LARGER

b. Define a subroutine GET_LARGER, darw the flowchart of the subroutine, and write the assembly code to implement this function.

;A program to compute the sum, difference,

;and absolute difference of two signed

;32-bit numbers.

;------Assembler Directives----------------

THUMB ; uses Thumb instructions

; Data Variables

AREA DATA, ALIGN=2 ; places objects in data memory (RAM)

EXPORT SUM [DATA,SIZE=4] ; export public varialbe "SUM" for use elsewhere

EXPORT DIFF [DATA,SIZE=4] ; export public varialbe "DIFF" for use elsewhere

EXPORT ABS [DATA,SIZE=4] ; export public varialbe "ABS" for use elsewhere

SUM SPACE 4 ; allocates 4 uninitialized bytes in RAM for SUM

DIFF SPACE 4 ; allocates 4 uninitialized bytes in RAM for DIFF

ABS SPACE 4 ; allocates 4 uninitialized bytes in RAM for ABS

; Code

AREA |.text|, CODE, READONLY, ALIGN=2 ; code in flash ROM

EXPORT Start ; export public function "start" for use elsewhere

NUM1 DCD -1 ; 32-bit constant data NUM1 = -1

NUM2 DCD 2 ; 32-bit constant data NUM2 = 2

;-------End of Assembler Directives----------

GET_SUM ; subroutine GET_SUM

ADD R0, R1, R2 ; R0=R1+R2

LDR R3, =SUM ; R3=&SUM, R3 points to SUM

STR R0, [R3] ; store the sum of NUM1 and NUM2 to SUM

BX LR ; subroutine return

GET_DIFF ; subroutine GET_DIFF

SUBS R0, R1, R2 ; R0=R1-R2

LDR R3, =DIFF ; R3=&DIFF, R3 points to DIFF

STR R0, [R3] ; store the different of NUM1 and NUM2 to DIFF

BMI GET_ABS ; check condition code, if N=1 (i.e. the difference is negative),

; branch to GET_ABS to calculate the absolute difference

STR_ABS ; label STR_ABS, store the absolute difference

LDR R3, =ABS ; R3=&ABS, R3 points to ABS

STR R0, [R3] ; store the absolute difference to ABS

BX LR ; subroutine return

GET_ABS ; label GET_ABS, calculate the absolute difference if the difference is negative

RSB R0, R0, #0 ; R0=0-R0;

B STR_ABS ; branch to STR_ABS to store the result

Start LDR R1, NUM1 ; R1=NUM1

LDR R2, NUM2 ; R2=NUM2

BL GET_SUM

BL GET_DIFF

ALIGN ; make sure the end of this section is aligned

END ; end of file

Explanation / Answer

Data Variables

AREA DATA, ALIGN=2 ; places objects in data memory (RAM)

EXPORT SUM [DATA,SIZE=4] ; export public varialbe "SUM" for use elsewhere

EXPORT DIFF [DATA,SIZE=4] ; export public varialbe "DIFF" for use elsewhere

EXPORT ABS [DATA,SIZE=4] ; export public varialbe "ABS" for use elsewhere

SUM SPACE 4 ; allocates 4 uninitialized bytes in RAM for SUM

DIFF SPACE 4 ; allocates 4 uninitialized bytes in RAM for DIFF

ABS SPACE 4 ; allocates 4 uninitialized bytes in RAM for ABS

; Code

AREA |.text|, CODE, READONLY, ALIGN=2 ; code in flash ROM

EXPORT Start ; export public function "start" for use elsewhere

NUM1 DCD -1 ; 32-bit constant data NUM1 = -1

NUM2 DCD 2 ; 32-bit constant data NUM2 = 2

;-------End of Assembler Directives----------

GET_SUM ; subroutine GET_SUM

ADD R0, R1, R2 ; R0=R1+R2

LDR R3, =SUM ; R3=&SUM, R3 points to SUM

STR R0, [R3] ; store the sum of NUM1 and NUM2 to SUM

BX LR ; subroutine return

GET_DIFF ; subroutine GET_DIFF

SUBS R0, R1, R2 ; R0=R1-R2

LDR R3, =DIFF ; R3=&DIFF, R3 points to DIFF

STR R0, [R3] ; store the different of NUM1 and NUM2 to DIFF

BMI GET_ABS ; check condition code, if N=1 (i.e. the difference is negative),

; branch to GET_ABS to calculate the absolute difference

STR_ABS ; label STR_ABS, store the absolute difference

LDR R3, =ABS ; R3=&ABS, R3 points to ABS

STR R0, [R3] ; store the absolute difference to ABS

BX LR ; subroutine return

GET_ABS ; label GET_ABS, calculate the absolute difference if the difference is negative

RSB R0, R0, #0 ; R0=0-R0;

B STR_ABS ; branch to STR_ABS to store the result

Start LDR R1, NUM1 ; R1=NUM1

LDR R2, NUM2 ; R2=NUM2

BL GET_SUM

BL GET_DIFF

ALIGN   

END   

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