A Marvel to Behold
If I remember correctly this beast had 256 bytes of memory. I breezed through Pascal and Fortran. I felt confident. And then this beauty, this 8-bit power house humbled, or more accurately, humilated me.
I haven't played with Assembly Language since and, as time heals all wounds, I forgot the pain. These two simple programs, written in NASM, were enough to sate my nastalgia.
The first is a simple hello world program. The second is a turn-based tic-tac-toe program. They're not much, but enough to sate my nostalgia.
Table of Contents
Hello World Example
This is my first NASM program, just a simple, getting started Hello World program. You can download the code here: helloworld.asm
; -------------------------------------------------------------------------.
section .text
org 100h
mov dx,my_message
mov ah,9 ; Write string to STDOUT, DS:DX=pointer to '$' terminated string
int 21h
mov al,0 ; return code (0 = no error)
mov ah,4Ch ; "EXIT" - TERMINATE WITH RETURN CODE, AL = return code
int 21h
section .data
my_message db 'Hello',10,'$
Tic Tac Toe Example
This is my second NASM program, just a simple, tic-tac-toe game. You can download the code here: tic-tac-toe.asm
; -------------------------------------------------------------------------.
section .text
; ========================================================================
; Main Program
; ========================================================================
org 100h
mov cl,[perform_intro]
;cmp cl,1
;and cl,cl
or cl,cl
;xor cl,cl
;inc cl
jz end_if1
mov dx,my_message
mov ah,9 ; Write string to STDOUT, DS:DX=pointer to '$' terminated string
int 21h
mov dx,separator
mov ah,9 ; Write string to STDOUT, DS:DX=pointer to '$' terminated string
int 21h
end_if1:
mov cx,9
mov dl,'x'
for_loop:
push cx
; mov cx,my_message_len_equ
; mov cx,[my_message_len_dw]
push dx
mov dx,prompt
call print_string_0
call input_user_move
pop dx
call update_board
call print_board
cmp dl,'x'
je mov_o
mov dl,'x'
jmp end_mov
mov_o:
mov dl,'o'
end_mov:
push dx
check_for_win:
mov si,row1
mov cx,3
check_for_win_row_loop:
push cx ; save loop counter
xor cx,cx ; clear cx
mov cl,[si] ; move contents of addess pointer into cl
mov ax,cx ; add new contents to accumlator ax
mov cl,[si+2] ; move contents of 2nd cell into cl
add ax,cx ; add to accumulator
mov cl,[si+4] ; move contents of 3rd cell into cl
add ax,cx ; add to accumulator
cmp ax,3*'o' ; check if 3 'o' were in the three rows
jz win_found
cmp ax,3*'x' ; check if 3 'x' were in the three rows
jz win_found
add si,8 ; move to next row (there 8bytes per row)
pop cx ; get loop counter
dec cx ; decrement loop counter. stay in loop if not zero
jnz check_for_win_row_loop ; stay in loop until cx is zero
mov si,row1
mov cx,3
check_for_win_col_loop:
push cx ; save loop counter
xor cx,cx ; clear cx
mov cl,[si] ; move contents of addess pointer into cl
mov ax,cx ; add new contents to accumlator ax
mov cl,[si+8] ; move contents from 2nd row into cl
add ax,cx ; add to accumulator
mov cl,[si+2*8] ; move contents from 3rd row into cl
add ax,cx ; add to accumulator
cmp ax,3*'o' ; check if 3 'o' were in the three rows
jz win_found
cmp ax,3*'x' ; check if 3 'x' were in the three rows
jz win_found
add si,2 ; move to next column (skip the vertical bar)
pop cx ; get loop counter
dec cx ; decrement loop counter. stay in loop if not zero
jnz check_for_win_col_loop ; stay in loop until cx is zero
mov si,row1
xor cx,cx ; clear cx
mov cl,[si] ; move contents of addess pointer into cl
mov ax,cx ; add new contents to accumlator ax
mov cl,[si+8+2] ; move contents from 2nd row into cl
add ax,cx ; add to accumulator
mov cl,[si+2*8+4] ; move contents from 3rd row into cl
add ax,cx ; add to accumulator
cmp ax,3*'o' ; check if 3 'o' were in the three rows
jz win_found
cmp ax,3*'x' ; check if 3 'x' were in the three rows
jz win_found
mov si,row1
xor cx,cx ; clear cx
mov cl,[si+4] ; move contents of addess pointer into cl
mov ax,cx ; add new contents to accumlator ax
mov cl,[si+8+2] ; move contents from 2nd row into cl
add ax,cx ; add to accumulator
mov cl,[si+2*8] ; move contents from 3rd row into cl
add ax,cx ; add to accumulator
cmp ax,3*'o' ; check if 3 'o' were in the three rows
jz win_found
cmp ax,3*'x' ; check if 3 'x' were in the three rows
jz win_found
pop dx
;mov dx,my_message
;call print_string_0
;print_string
;mov ah,40h
;mov bx,01h ; stdout
;int 21h
pop cx
dec cx
jnz for_loop
win_found:
cmp ax,3*'o'
jz o_wins
mov dx,x_is_the_winner
call print_string_0
jmp end_print_winner
o_wins:
mov dx,o_is_the_winner
call print_string_0
end_print_winner:
mov al,0 ; return code (0 = no error)
mov ah,4Ch ; "EXIT" - TERMINATE WITH RETURN CODE, AL = return code
int 21h
; ========================================================================
; Functions and subroutines
; ========================================================================
print_string:
mov ah,40h
mov bx,01h ; stdout
int 21h
ret
print_string_0:
mov si,dx ; load the start of string into si
print_string_0_loop:
mov cl,[si] ; load the character byte into ch
inc si ; advance dx pointer to the next character
and cl,cl ; check to see if the char is zero
jnz print_string_0_loop
dec si ; si is pointing to the byte after null. decrement it back to null char.
sub si,dx ; subract first char address from last char address to get string length
mov cx,si ; int 21h ah=40h expects CX to contain the string length
mov ah,40h
mov bx,01h ; stdout
int 21h
ret ; print_string_0
input_user_move:
; read first char (row_id) from stdin without echo. The result is stored in AL
mov ah,07h
int 21h
sub al,30h ; convert ASCII to integer by subtracting '0'
mov [user_move],al
; read another char (column_id) from stdin without echo. The result is stored in AL
mov ah,07h
int 21h
sub al,30h ; convert ASCII to integer by subtracting '0'
mov [user_move+1],al
ret ;input_user_move
; 'x' or 'o' passed in with DL register
; ax, cx and si are corrupted
update_board:
xor ax,ax
mov al,[user_move+1]
shl ax,1 ;multiple by 2 to handle the vertical bar separators
xor cx,cx
mov cl,[user_move]
shl cx,3 ; multiply by 8 to get the row(each row is conviently 8 bytes)
add ax,cx
add ax,row1
mov si,ax
mov [si],dl
ret ;update_board
print_board:
push dx
mov dx,row1
call print_string_0
mov dx,sep
call print_string_0
mov dx,row2
call print_string_0
mov dx,sep
call print_string_0
mov dx,row3
call print_string_0
pop dx
ret; print_board
section .data
; ========================================================================
; Constants, Variables and Data Structures
; ========================================================================
perform_intro dw 0
my_message db 'He$$lloh ',41h,7,'$',10,0
my_message_len_equ equ $-my_message
my_message_len_dw dw my_message_len_equ
separator db 10,10,"------------------------",10,10,'$'
o_is_the_winner db 'Way to go O!',10,0
x_is_the_winner db 'X Rules!',10,0
prompt db 'Your move (00=top left, 02=top right, 11=middle, 22=lower right...)',10,0
user_move db 0,0
row1 db ' | | ',10,0
row2 db ' | | ',10,0
row3 db ' | | ',10,0
sep db '----- ',10,0
