name "convert"

; this program uses a subroutine written in 8086 assembly language  
; that can be used for converting a string of number  
; (max of 4 ascii digit) to equivalent packed bcd digits. 
; bcd is binary coded decimal. 

; this program does no screen output. 
; to see results click "vars". 




org 100h

jmp start

string db '1234'  ; 4 ascii digits. 
packed_bcd dw ?   ; two bytes (word) to store 4 digits. 

start:

lea bx, string
lea di, packed_bcd
call pack_to_bcd_and_binary

ret ; return to operating system. 





; subroutine written in 8086 assembly language  
; that can be used for converting a string of number  
; (max of 4 ascii digit) to equivalent packed 
; bcd digits.  
; input parameters: 
;    bx - address of source string (4 ascii digits). 
; output: 
;    di - must be set to address for packed bcd (2 bytes). 


pack_to_bcd_and_binary proc near
pusha

; point to 2 upper digits of packed bcd: 
; (assumed that we have 4 digits) 
add di, 1

; loop only for 2 because every time we 
; read 2 digits (2 x 2 = 4 digits) 
mov cx, 2

; reset packed bcd: 
mov word ptr [di], 0

	; to convert a char (0..9) to digit we need 
	; to subtract 48 (30h) from its ascii code, 
	; or just clear the upper nibble of a byte. 
	; mask:  00001111b  (0fh) 

next_digit:
mov ax, [bx]	; read 2 digits. 

and ah, 00001111b
and al, 00001111b

; 8086 and all other Intel's microprocessors store less  
; significant byte at lower address. 

xchg al, ah

; move first digit to upper nibble: 
shl ah, 4

; pack bcd: 
or ah, al

; store 2 digits: 
mov [di], ah

; next packed bcd: 
sub di, 1
; next word (2 digits): 
add bx, 2

loop next_digit

popa
ret
pack_to_bcd_and_binary endp