Assembly : Program Calculator

name “calc2”

; command prompt based simple calculator (+,-,*,/) for 8086.
; example of calculation:
; input 1 <- number: 10 ; input 2 <- operator: – ; input 3 <- number: 5 ; ——————- ; 10 – 5 = 5 ; output -> number: 5

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; this maro is copied from emu8086.inc ;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; this macro prints a char in AL and advances
; the current cursor position:
PUTC MACRO char
PUSH AX
MOV AL, char
MOV AH, 0Eh
INT 10h
POP AX
ENDM
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

org 100h

jmp start

; define variables:

msg0 db “note: calculator works with integer values only.”,0Dh,0Ah
db “to learn how to output the result of a float division see float.asm in examples”,0Dh,0Ah,’$’
msg1 db 0Dh,0Ah, 0Dh,0Ah, ‘enter first number: $’
msg2 db “enter the operator: + – * / : $”
msg3 db “enter second number: $”
msg4 db 0dh,0ah , ‘the approximate result of my calculations is : $’
msg5 db 0dh,0ah ,’thank you for using the calculator! press any key… ‘, 0Dh,0Ah, ‘$’
err1 db “wrong operator!”, 0Dh,0Ah , ‘$’
smth db ” and something…. $”

; operator can be: ‘+’,’-‘,’*’,’/’ or ‘q’ to exit in the middle.
opr db ‘?’

; first and second number:
num1 dw ?
num2 dw ?

start:
mov dx, offset msg0
mov ah, 9
int 21h

lea dx, msg1
mov ah, 09h ; output string at ds:dx
int 21h

; get the multi-digit signed number
; from the keyboard, and store
; the result in cx register:

call scan_num

; store first number:
mov num1, cx

; new line:
putc 0Dh
putc 0Ah

lea dx, msg2
mov ah, 09h ; output string at ds:dx
int 21h

; get operator:
mov ah, 1 ; single char input to AL.
int 21h
mov opr, al

; new line:
putc 0Dh
putc 0Ah

cmp opr, ‘q’ ; q – exit in the middle.
je exit

cmp opr, ‘*’
jb wrong_opr
cmp opr, ‘/’
ja wrong_opr

; output of a string at ds:dx
lea dx, msg3
mov ah, 09h
int 21h

; get the multi-digit signed number
; from the keyboard, and store
; the result in cx register:

call scan_num

; store second number:
mov num2, cx

lea dx, msg4
mov ah, 09h ; output string at ds:dx
int 21h

; calculate:

cmp opr, ‘+’
je do_plus

cmp opr, ‘-‘
je do_minus

cmp opr, ‘*’
je do_mult

cmp opr, ‘/’
je do_div

; none of the above….
wrong_opr:
lea dx, err1
mov ah, 09h ; output string at ds:dx
int 21h

exit:
; output of a string at ds:dx
lea dx, msg5
mov ah, 09h
int 21h

; wait for any key…
mov ah, 0
int 16h

ret ; return back to os.

do_plus:

mov ax, num1
add ax, num2
call print_num ; print ax value.

jmp exit

do_minus:

mov ax, num1
sub ax, num2
call print_num ; print ax value.

jmp exit

do_mult:

mov ax, num1
imul num2 ; (dx ax) = ax * num2.
call print_num ; print ax value.
; dx is ignored (calc works with tiny numbers only).

jmp exit

do_div:
; dx is ignored (calc works with tiny integer numbers only).
mov dx, 0
mov ax, num1
idiv num2 ; ax = (dx ax) / num2.
cmp dx, 0
jnz approx
call print_num ; print ax value.
jmp exit
approx:
call print_num ; print ax value.
lea dx, smth
mov ah, 09h ; output string at ds:dx
int 21h
jmp exit

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; these functions are copied from emu8086.inc ;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; gets the multi-digit SIGNED number from the keyboard,
; and stores the result in CX register:
SCAN_NUM PROC NEAR
PUSH DX
PUSH AX
PUSH SI

    MOV     CX, 0

    ; reset flag:
    MOV     CS:make_minus, 0

next_digit:

    ; get char from keyboard
    ; into AL:
    MOV     AH, 00h
    INT     16h
    ; and print it:
    MOV     AH, 0Eh
    INT     10h

    ; check for MINUS:
    CMP     AL, '-'
    JE      set_minus

    ; check for ENTER key:
    CMP     AL, 0Dh  ; carriage return?
    JNE     not_cr
    JMP     stop_input

not_cr:

    CMP     AL, 8                   ; 'BACKSPACE' pressed?
    JNE     backspace_checked
    MOV     DX, 0                   ; remove last digit by
    MOV     AX, CX                  ; division:
    DIV     CS:ten                  ; AX = DX:AX / 10 (DX-rem).
    MOV     CX, AX
    PUTC    ' '                     ; clear position.
    PUTC    8                       ; backspace again.
    JMP     next_digit

backspace_checked:

    ; allow only digits:
    CMP     AL, '0'
    JAE     ok_AE_0
    JMP     remove_not_digit

ok_AE_0:
CMP AL, ‘9’
JBE ok_digit
remove_not_digit:
PUTC 8 ; backspace.
PUTC ‘ ‘ ; clear last entered not digit.
PUTC 8 ; backspace again.
JMP next_digit ; wait for next input.
ok_digit:

    ; multiply CX by 10 (first time the result is zero)
    PUSH    AX
    MOV     AX, CX
    MUL     CS:ten                  ; DX:AX = AX*10
    MOV     CX, AX
    POP     AX

    ; check if the number is too big
    ; (result should be 16 bits)
    CMP     DX, 0
    JNE     too_big

    ; convert from ASCII code:
    SUB     AL, 30h

    ; add AL to CX:
    MOV     AH, 0
    MOV     DX, CX      ; backup, in case the result will be too big.
    ADD     CX, AX
    JC      too_big2    ; jump if the number is too big.

    JMP     next_digit

set_minus:
MOV CS:make_minus, 1
JMP next_digit

too_big2:
MOV CX, DX ; restore the backuped value before add.
MOV DX, 0 ; DX was zero before backup!
too_big:
MOV AX, CX
DIV CS:ten ; reverse last DX:AX = AX*10, make AX = DX:AX / 10
MOV CX, AX
PUTC 8 ; backspace.
PUTC ‘ ‘ ; clear last entered digit.
PUTC 8 ; backspace again.
JMP next_digit ; wait for Enter/Backspace.

stop_input:
; check flag:
CMP CS:make_minus, 0
JE not_minus
NEG CX
not_minus:

    POP     SI
    POP     AX
    POP     DX
    RET

make_minus DB ? ; used as a flag.
SCAN_NUM ENDP

; this procedure prints number in AX,
; used with PRINT_NUM_UNS to print signed numbers:
PRINT_NUM PROC NEAR
PUSH DX
PUSH AX

    CMP     AX, 0
    JNZ     not_zero

    PUTC    '0'
    JMP     printed

not_zero:
; the check SIGN of AX,
; make absolute if it’s negative:
CMP AX, 0
JNS positive
NEG AX

    PUTC    '-'

positive:
CALL PRINT_NUM_UNS
printed:
POP AX
POP DX
RET
PRINT_NUM ENDP

; this procedure prints out an unsigned
; number in AX (not just a single digit)
; allowed values are from 0 to 65535 (FFFF)
PRINT_NUM_UNS PROC NEAR
PUSH AX
PUSH BX
PUSH CX
PUSH DX

    ; flag to prevent printing zeros before number:
    MOV     CX, 1

    ; (result of "/ 10000" is always less or equal to 9).
    MOV     BX, 10000       ; 2710h - divider.

    ; AX is zero?
    CMP     AX, 0
    JZ      print_zero

begin_print:

    ; check divider (if zero go to end_print):
    CMP     BX,0
    JZ      end_print

    ; avoid printing zeros before number:
    CMP     CX, 0
    JE      calc
    ; if AX<BX then result of DIV will be zero:
    CMP     AX, BX
    JB      skip

calc:
MOV CX, 0 ; set flag.

    MOV     DX, 0
    DIV     BX      ; AX = DX:AX / BX   (DX=remainder).

    ; print last digit
    ; AH is always ZERO, so it's ignored
    ADD     AL, 30h    ; convert to ASCII code.
    PUTC    AL


    MOV     AX, DX  ; get remainder from last div.

skip:
; calculate BX=BX/10
PUSH AX
MOV DX, 0
MOV AX, BX
DIV CS:ten ; AX = DX:AX / 10 (DX=remainder).
MOV BX, AX
POP AX

    JMP     begin_print

print_zero:
PUTC ‘0’

end_print:

    POP     DX
    POP     CX
    POP     BX
    POP     AX
    RET

PRINT_NUM_UNS ENDP

ten DW 10 ; used as multiplier/divider by SCAN_NUM & PRINT_NUM_UNS.

GET_STRING PROC NEAR
PUSH AX
PUSH CX
PUSH DI
PUSH DX

MOV CX, 0 ; char counter.

CMP DX, 1 ; buffer too small?
JBE empty_buffer ;

DEC DX ; reserve space for last zero.

;============================
; Eternal loop to get
; and processes key presses:

wait_for_key:

MOV AH, 0 ; get pressed key.
INT 16h

CMP AL, 0Dh ; ‘RETURN’ pressed?
JZ exit_GET_STRING

CMP AL, 8 ; ‘BACKSPACE’ pressed?
JNE add_to_buffer
JCXZ wait_for_key ; nothing to remove!
DEC CX
DEC DI
PUTC 8 ; backspace.
PUTC ‘ ‘ ; clear position.
PUTC 8 ; backspace again.
JMP wait_for_key

add_to_buffer:

    CMP     CX, DX          ; buffer is full?
    JAE     wait_for_key    ; if so wait for 'BACKSPACE' or 'RETURN'...

    MOV     [DI], AL
    INC     DI
    INC     CX

    ; print the key:
    MOV     AH, 0Eh
    INT     10h

JMP wait_for_key
;============================

exit_GET_STRING:

; terminate by null:
MOV [DI], 0

empty_buffer:

POP DX
POP DI
POP CX
POP AX
RET
GET_STRING ENDP