Implementation of partyhard mode in x86_16 asm.

partytxt.asm

; A text-mode (well, 640x480 16 colours VGA graphics mode hosting 80x25 text)
; party hard mode in 16-bit x86 asm.  Requires a vga graphics adapter that isn't
; fussy, as we make a mess of various things and ignore other things because we
; don't care and want to save space.
;
; WARNING: IF YOU SUFFER FROM EPILEPSY OR OTHER PHOTOSENSITIVITY DISORDER, DO
; NOT RUN THIS PROGRAM OR VIEW IT'S OUTPUT, AS IT WILL BE HARMFUL TO YOUR
; HEALTH.
; 
; Compile with nasm -fbin partytxt.asm -o partytxt.com
;
; Michael Farrell <micolous+asm at gmail dot com>, 2010-04-06
org 100h

; store the random seed after the palette in unallocated memory
%define r_seed palette + 18

_start:
	; stop ^c
	cli

	; set 640x480x16 colours
	mov ax, 0012h ; ah=00h, al=12h
	int 10h
	
	;xor bl, bl
	.loop:
		; putparty
		; put the text on the screen in a specific colour (bl), character by character
		lea di, [partymsg]
		mov ah, 0Eh
		;xor bh, bh
		.ploop:
			mov al, [di]
			int 10h
		
			inc di
			;cmp byte [di], 0
			cmp di, partymsg.end
			
			jl .ploop

		call random
		;mov bx, word [random.r_seed]
		mov bx, dx  ; set a random colour, this is later used for the text colour
		            ; as well
		; set overscan colour
		mov ax, 1001h
		int 10h

		jmp .loop
		; set the rest of the pallete
		lea si, [palette]
		;xor dl, dl
		.palloop:
			call random
			;mov ax, word [random.r_seed]

			; make sure it's all nonzero
			cmp dh, 0
			je .palloop
			cmp dl, 0
			je .palloop
			
			mov [si], dx ; dx in this context is still set to the value of the random
			             ; number as the random function doesn't cleanup
			             
			inc si ; Increment the position in the palette we're writing to
			inc si ; This takes two less bytes than `add si, 2`.
			
			cmp si, palette + 16
			jne .palloop
				
		; wait for refresh
		mov dx,03dah
		in al,dx
		;test al,8
		cmp al, 7
		;jz $-3
		jg .loop
		
		in al,dx
		test al,8
		jnz .loop
		
		; pallete generated, push it
		mov ax, 1002h
		;xor bh, bh
		mov dx, [palette]
		int 10h
		
		jmp .loop
		
;	.endloop:
;		; read a character again and again
;		mov ah, 08h
;		int 21h
;		jmp .endloop

partymsg:
	dd "PARTY HARD! " ;, 0
	;dd "YOU HAVE JUST LOST THE GAME! "
	.end:

random: 
	; Generates a random number in dx and r_seed.  Take your pick which one you
	; want to use.
	;
	; Stolen from ftp://ftp.simtel.net/pub/simtelnet/msdos/asmutl/alib40.zip
	; /alib4c.zip/random7.asm
	; And ported to nasm, and squished a little bit because we don't care about
	; making a big mess
	
	;push dx
	;push ax
	mov ax, [r_seed]
	mul word [.r_multipler]
	;add	ax, 0FFFFh ;[.r_increment]
	dec ax ; has the same effect, right?
	
	;adc	dx, 0    ; Unsure what effect this has, but "add with carry" a value of
	               ; 0 sounds like it does absolutely nothing.  I removed it and
	               ; it seems to have no effect on the randomness in this context.
	div	word [.r_permute1]
	;xchg ax,dx
	mov	[r_seed], dx ;ax
	;pop ax
	;pop	dx
	ret
	
	;.r_seed		dw	00000h		; random number seed / random result
	.r_permute1	dw	0FFEFh		;  modulus	2^16-17
	.r_multipler	dw	0FFD9h		;  multiplier	2^16-39
	;.r_increment	dw	0FFFFh		;  increment	-1

palette:
 ; 18 bytes used here for the colour palette
 ; 2 bytes used after that for the random seed