How to preserve an array or string in x86

Question

I'm programming the game "15 puzzle game" in x86 using tasm asm/dosbox. I want to "save" or "preserve" my array called 'a' that is being declared in my data segment, so that after I swap the bytes around using upArrow, downArrow, etc... and then I press 'n' for new game, I can set my array 'a' back to it's original state as it was declared in my data segment. Here is my code:

.MODEL TINY
.386

.DATA
PROMPT DB "Enter q to quit, arrow keys to move, n to restart (new game) HAPPY HOLIDAYS!! :)$"

;this is board preset A
a       DB 201,205,205,205,205,203,205,205,205,205,203,205,205,205,205,203,205,205,205,205,203,205,205,205,205,203,205,205,205,205,187,
    DB 186,255,48,50,255,186,255,48,49,255,186,255,48,51,255,186,255,48,52,255,186,255,48,53,255,186,255,48,54,255,186,
    DB 204,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,185,
    DB 186,255,48,55,255,186,255,48,56,255,186,255,48,57,255,186,255,49,48,255,186,255,49,49,255,186,255,49,50,255,186,
    DB 204,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,185, 
    DB 186,255,49,51,255,186,255,49,52,255,186,255,49,53,255,186,255,49,54,255,186,255,49,55,255,186,255,255,255,255,186,
    DB 204,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,206,205,205,205,205,185, 
    DB 186,255,49,56,255,186,255,49,57,255,186,255,50,48,255,186,255,50,49,255,186,255,50,50,255,186,255,50,51,255,186,
    DB 200,205,205,205,205,202,205,205,205,205,202,205,205,205,205,202,205,205,205,205,202,205,205,205,205,202,205,205,205,205,188,"$"


col DB 0
row DB 0
board DB 0

.CODE
org 100h
MAIN PROC

;initialize display step 0
MOV AH, 00h
MOV AL, 03h
INT 10h

MOV AX, 0B800h
MOV ES, AX
XOR di,di


;initialize configuration and dislay prompt step 1
newGame:
call initConfig

;display board step 2


;wait for key step 3
game:
;print board
call printBoard1

;get cursor x y
call SetCursor

MOV AH, 0
INT 16H

MOV BL, AL

;up arrow
CMP AH, 48h
jz upArrow

;down arrow
CMP AH, 50h
jz downArrow

;right arrow
CMP AH, 4dh
jz rightArrow

;left arrow
CMP AH, 4bh
jz leftArrow

;lowercase q
CMP AL, 71h
jz EXIT

;uppercase q
CMP AL, 51h
jz EXIT

;lowercase n
CMP AL, 6eh
jz newGame

;uppercase n
CMP AL, 4eh
jz newGame

jmp game

MAIN ENDP

EXIT:
MOV AH, 4CH                  ; return control to DOS
INT 21H

SetCursor:
mov dl, col
mov dh, row
mov bh, 0
mov ah, 02h
int 10h
ret

getStrlen:
lodsb
cmp al, 24h
je strlen_end
inc bx
jmp getStrlen
strlen_end:
ret

loadAndDisplayStr:
lodsb
stosw
dec cx
jne loadAndDisplayStr
ret

printBoard1:
MOV board, 1
xor dx,dx
xor ax,ax
mov cx, 9
myloop1:
push cx
lea si, a
add si, dx
mov cx, 31
mov di, ax
push ax
mov ah, 4
call loadAndDisplayStr
pop ax
add ax, 160
add dx, 32
pop cx
dec cx
jnz myloop1
ret

upArrow:
xor si, si
xor di, di
lea bx, a
mov ax, 32
sub row, 2
mul row
add al, col
mov di, ax
mov ax, 32
add row, 2
mul row
add al, col
mov si, ax
mov dx, [bx+si]
xchg dx, [bx+di]
xchg [bx+si], dx
sub row, 2
jmp game

downArrow:
xor si, si
xor di, di
lea bx, a
mov ax, 32
add row, 2
mul row
add al, col
mov di, ax
mov ax, 32
sub row, 2
mul row
add al, col
mov si, ax
mov dx, [bx+si]
xchg dx, [bx+di]
xchg [bx+si], dx
add row, 2
jmp game

leftArrow:
xor si, si
xor di, di
lea bx, a
mov ax, 32
sub col, 5
mul row
add al, col
mov di, ax
mov ax, 32
add col, 5
mul row
add al, col
mov si, ax
mov dx, [bx+si]
xchg dx, [bx+di]
xchg [bx+si], dx
sub col, 5
jmp game

rightArrow:
xor si, si
xor di, di
lea bx, a
mov ax, 32
add col, 5
mul row
add al, col
mov di, ax
mov ax, 32
sub col, 5
mul row
add al, col
mov si, ax
mov dx, [bx+si]
xchg dx, [bx+di]
xchg [bx+si], dx
add col, 5
jmp game

initConfig:
;put the cursor at 00
mov col, 27
mov row, 5

;gets strlen of prompt and stores it in BX
lea si, PROMPT
xor bx, bx
call getStrlen

;display prompt
lea si, PROMPT
mov cx, bx
mov ah, 2
mov di, 5a0h
call loadAndDisplayStr
ret

END MAIN

The way I see it is there are 2 ways of doing this. One way would be to create an empty array, and every time the user makes a move (up/down/left/right) I can push this to a stack, and then when the user hits 'n' for new game, I just pop through that stack and reverse all the moves.

Another alternative would be to create 2 identical boards (arrays/string) 'a' and 'b', and then the user would manipulate board 'a' till they decide to hit 'n' for a new game, and then I would some how set 'a' = 'b'

Answer 1

There's no magic way, you just have to copy the array like you would in C. I'd suggest having one master copy of the data that you keep clean / untouched, never ever writing into it.

Allocate uninitialized space for the working game state somewhere, eg on the stack or in the BSS ¹ .

At the start of every game (including the first one), copy the preset whole array to the scratch space for the current game. (eg with rep movsb or rep movsw after setting CX, SI, and DI. ES and DS segment regs are already equal segment because of.model tiny. ie implement a memcpy however you like.)

So you don't need to write the initializer data twice or anything like that, and only one copy of it needs to be in the file.

And no you don't need any exchanging, just a block copy. Get the assembler to calculate the size of the whole thing by putting a label at the end, or put datasize = $ - a right after the end of a to get the size in bytes.

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Footnote 1 : See @MichaelPetch's comment for details on how to declare a label in the .data? segment so you can use normal label/symbol syntax. Like gamestate datasize dup(?) , where datasize is an = or equ constant you got the assembler to calculate earlier from the size of the preset.

I think .data? is just space past the end of where DOS loads your .com file, and you can use space from there out to near the end of a 64k segment where the program-loader put your stack. (Unlike the.bss under a modern OS is not zeroed to start with. That's fine for this, you want to write the space you use before you read/modify it anyway.)

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@DavidWohlferd also suggests you could re-read part of the executable from disk, with DOS file open/read system calls. That could be the start of a save-game mechanism. In its simplest form, if the first n bytes of your program are all read-only (code and constant data) apart from this game-state array, you could read the first n bytes of the raw .com file into memory at ds:100h (or cs or es, it's all the same in a tiny memory model). Overwriting instructions with themselves (like a jmp rel16 over the data) is fine.