lab 2 subtracting calculator

As part of my lab 2 of using 6502 assembly, i have decided to work on a subtracting calculatror. It was based on a simple adding calculator provided. It prompted the user to enter two numbers, added them together and display the results. One of the main challenges was when changed from adding to subtracting calculator, it was the second number subtracts the first number input, so I have to make more changes to ensuring that the first number was subtracted from the second number and not the other way around. Another challenge was handling binary-coded decimal subtraction correctly. The initial attempts resulted in incorrect results, which were quite puzzling. The one minnor issue remaining is I have to hit enter twice for each input.


 

; Subtracting calculator


; ROM routine entry points

define SCINIT $ff81 ; initialize/clear screen

define CHRIN $ffcf ; input character from keyboard

define CHROUT $ffd2 ; output character to screen

define SCREEN $ffed ; get screen size

define PLOT $fff0 ; get/set cursor coordinates


; zeropage variables

define PRINT_PTR $10

define PRINT_PTR_H $11

define value1 $14

define value1_h $15

define value2 $16

define value2_h $17


; absolute variables

define GETNUM_1 $0080

define GETNUM_2 $0081


; constants


; --------------------------------------------------------


jsr SCINIT

jsr CHRIN


jsr PRINT


dcb "S","u","b","t","r","a","c","t","i","n","g",32

dcb "c","a","l","c","u","l","a","t","o","r",00


start: jsr PRINT

dcb $0d,$0d,"E","n","t","e","r",32,"a",32,"n","u","m","b","e","r"

dcb "(","0","-","9","9",")",":"

dcb 32,32,32,32,32,32,32,32,00


lda #$00

sta value1_h

sta value2_h


jsr GETNUM

sta value1


jsr PRINT


dcb "E","n","t","e","r",32,"a","n","o","t","h","e","r"

dcb 32,"n","u","m","b","e","r",32,"(","0","-","9","9",")",":",32,00


jsr GETNUM

sed

sta value2

sec

lda value1

sbc value2

cld


sta value1

bcs result

dec value1_h


result: pha

jsr PRINT


dcb "R","e","s","u","l","t",":",32

dcb 32,32,32,32,32,32,32

dcb 32,32,32,32,32,32,32

dcb 32,32,32,32,32,32,32

dcb 00


lda value1_h

beq low_digits

lda #$31

jsr CHROUT

jmp draw_100s


low_digits: lda value1

and #$f0

beq ones_digit


draw_100s: lda value1

lsr

lsr

lsr

lsr

ora #$30

jsr CHROUT


ones_digit: lda value1

and #$0f

ora #$30

jsr CHROUT


jsr start


; --------------------------------------------------------

; Print a message

; 

; Prints the message in memory immediately after the 

; JSR PRINT. The message must be null-terminated and

; 255 characters maximum in length.


PRINT: pla

clc

adc #$01

sta PRINT_PTR

pla

sta PRINT_PTR_H


tya

pha


ldy #$00

print_next: lda (PRINT_PTR),y

beq print_done

jsr CHROUT

iny

jmp print_next


print_done: tya

clc

adc PRINT_PTR

sta PRINT_PTR


lda PRINT_PTR_H

adc #$00

sta PRINT_PTR_H


pla

tay


lda PRINT_PTR_H

pha

lda PRINT_PTR

pha


rts


; ---------------------------------------------------

; GETNUM - get a 2-digit decimal number

;

; Returns A containing 2-digit BCD value


GETNUM: txa

pha

tya

pha


ldx #$00 ; count of digits received

stx GETNUM_1

stx GETNUM_2



getnum_cursor: lda #$a0 ; black square

jsr CHROUT

lda #$83 ; left cursor

jsr CHROUT


getnum_key: jsr CHRIN

cmp #$00

beq getnum_key


cmp #$08 ; BACKSPACE

beq getnum_bs


cmp #$0d ; ENTER

beq getnum_enter


cmp #$30 ; "0"

bmi getnum_key


cmp #$3a ; "9" + 1

bmi getnum_digit


jmp getnum_key


getnum_enter: cpx #$00

beq getnum_key


lda #$20

jsr CHROUT

lda #$0d

jsr CHROUT


lda GETNUM_1


cpx #$01

beq getnum_done


asl

asl

asl

asl

ora GETNUM_2


getnum_done: sta GETNUM_1

pla

tay

pla

tax

lda GETNUM_1


rts


getnum_digit: cpx #$02

bpl getnum_key

pha

jsr CHROUT

pla

and #$0f

sta GETNUM_1,x

inx

jmp getnum_cursor


getnum_bs: cpx #$00

beq getnum_key

lda #$20

jsr CHROUT

lda #$83

jsr CHROUT

jsr CHROUT

lda #$20

jsr CHROUT

lda #$83

jsr CHROUT

dex

lda #$00

sta GETNUM_1,x

jmp getnum_cursor


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