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‎SC.ASM

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;-----------------------------------------------------------------------:
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; A true matrix screensaver, this version works ONLY in bw modes :
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; Use command tail in PSP as stack and local var space :
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; Written by: Yll Buzoku :
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; Date: 27/08/2021 :
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;-----------------------------------------------------------------------:
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CODE SEGMENT
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ASSUME CS:CODE, DS:CODE, ES:CODE, SS:CODE
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;-----------------------------------------------------------------------:
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; PSP equates area. :
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;-----------------------------------------------------------------------:
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ORG 0FFh
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VARS PROC NEAR
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stkend db ? ;Stack tail, max 60 bytes, go into FCB area
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seed dw ? ;RNG seed value, overlay jmp TRANS
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inFlag db ? ;Bits [7:1] reserved, Bit [0]-inTSR flag
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VARS ENDP
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;-----------------------------------------------------------------------:
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ORG 100H
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EP PROC NEAR
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jmp TRANS ;Just save a word by recycling this instruction
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curs dw ? ;Saves BDA cursor word before modification
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ticks dw ? ;Timeout comparator var, 0888h is 2 minutes
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ogss dw ? ;Caller stack segment
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ogsp dw ? ;Caller stack pointer
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EP ENDP
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TSRMAIN PROC NEAR
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;Saves current screen and starts scrolling ascii chars down the screen.
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;First manually set segregs
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cli
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mov word ptr cs:[ogsp], sp
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mov sp, ss
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mov word ptr cs:[ogss], sp
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mov sp, cs ;Move code segment into ss
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mov ss, sp
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mov sp, word ptr cs:[stkend] ;Point sp to offset of the bottom of stk
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sti
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push ax
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push bx
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push cx
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push dx
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push ds
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push es
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push si
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push di
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;Reset segvars
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push cs
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pop ax
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mov ds, ax
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mov es, ax
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;Now detect video mode
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;mov ah, 0Fh
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;int 10h
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;Returns al=Vid mode, ah=#of cols, bh=active page
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;mov byte ptr [mode], al
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;mov byte ptr [cols], ah
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;mov byte ptr [page], bh
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;mov word ptr [regseg], 0b800h ;Assume colour card
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;cmp al, 07h ;Are we a BW card, currently active?
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;jne i1
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;sub word ptr [regseg], 800h ;Move it back to bw space
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i1:
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;First we create a first seed
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xor ah, ah
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int 1Ah
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mov word ptr [seed], dx ;Get ticks into seed var
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xor bh, bh ;0 page
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mov ah, 03h ;read cursor pos
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int 10h
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mov word ptr [curs], dx ;Save cursor pos
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; mov ax, word ptr [regseg] ;Get actual segment
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mov ax, 0b000h ;Load bw seg
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push ds
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mov ds, ax ;Set ds to point to bios regen
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xor si, si
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mov di, OFFSET regen ;es points to current code segment
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mov cx, 2000 ;2000 words to be transferred
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rep movsw
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pop ds ;Get original ds back
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;Buffers copied, time for some fun!
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iloop: ;Main loop of this meme
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; mov bl, byte ptr [cols]
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mov bl, 80 ;80 columns, hardcoded
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call rndgen
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mov al, bl ;Move the random column into al
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;Get a random column function
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mov bl, 0FFh ;Sub function options
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call rndgen
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mov ah, bl ;Move random column function into ah
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call col
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test byte ptr [inFlag], 1 ;Will only be zero if key pressed
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jnz iloop
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iexit:
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;Exit below here. Return regen to regseg
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push cs
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pop ds
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; mov ax, word ptr [regseg]
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mov ax, 0b000h ;bw seg
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mov es, ax
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mov si, OFFSET regen
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xor di, di
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mov cx, 2000
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rep movsw
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mov dx, word ptr [curs] ;Get original cursor position back
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xor bh, bh ;0 page
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mov ah, 02h ;set cursor
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int 10h
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pop di
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pop si
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pop es
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pop ds
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pop dx
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pop cx
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pop bx
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pop ax
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cli
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mov sp, word ptr cs:[ogss]
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mov ss, sp
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mov sp, word ptr cs:[ogsp]
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sti
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iret ;Alternate exit point, when exiting from screensaver
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TSRMAIN ENDP
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IRQ0 PROC FAR ;Timer replacement
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;Timer initiates whether or not to go into screensaver
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;Wraps old int 09h, to continue gracefully after hardware EOI
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jmp short irq0in
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dw 55AAh ;Installed signature
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irq0in:
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pushf ;Fake interrupt call, could just inc sp
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db 09Ah ;Far Call, manually encoded
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irq0o: dw ?
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irq0s: dw ?
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test byte ptr cs:[inFlag], 1
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jnz irq0cont ;If its 1, dont inc ticks, proceed as normal
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inc word ptr cs:[ticks]
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;DEBUG DEBUG DEBUG DEBUG DEBUG
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; cmp word ptr cs:[ticks], 0B6h ;Temp 10 seconds 0888h
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;DEBUG DEBUG DEBUG DEBUG DEBUG
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cmp word ptr cs:[ticks], 0888h
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jb irq0cont
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inc byte ptr cs:[inFlag] ;Set bit 0, indicate going into TSR
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mov word ptr cs:[ticks], 0
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jmp tsrmain
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irq0cont:
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iret
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IRQ0 ENDP
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IRQ1 PROC FAR ;Keyb replacement
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mov word ptr cs:[ticks], 0 ;Null out the ticks counter
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mov byte ptr cs:[inFlag],0
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db 0EAh ;Far jump to original interrupt handler
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irq1o: dw ? ;Var space for og handler/part of opcode
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irq1s: dw ?
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IRQ1 ENDP
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RNDGEN PROC NEAR
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;----------------------------------------------------
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; Generates a random 8 bit value and returns it. :
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; Input: bl = Modulo value (max value plus 1). :
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; Ret: bl = Random return value. :
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; All registers preserved. :
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;----------------------------------------------------
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push ax
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push cx
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push dx
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mov ax, 25173 ;Random multiplier
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mul word ptr [seed] ;Multiply to seed
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add ax, 13849
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mov word ptr [seed], ax
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;ax now has new random value
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mov cl, bl ;Get max number in cl
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xor ch, ch
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xor dx, dx ;Clear upper word for division for no overflow?
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div cx ;Divide ax by cl
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mov bl, dl ;move remainder into bl
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pop dx
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pop cx
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pop ax
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ret
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RNDGEN ENDP
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COL PROC NEAR
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;-----------------------------------------------------
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; Takes column to scroll down, and scrolls it down. :
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; Might generate a new char or even clear column. :
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; Input: al=Column number (0-49), ah=Bit 7 :
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; ah(7) clear = Scroll column :
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; ah(7) set = Add new char + scroll :
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; Output: Nothing, ax destroyed. :
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;-----------------------------------------------------
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push dx
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push cx
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push bx
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push si
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xor ch, ch ;Upper row
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mov dh, 24 ;Lower row
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mov cl, al ;Column to scroll in cl
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mov dl, al ;Column to scroll in dl
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mov si, ax ;Save col number in si
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mov bx, 0700h ;Usual char attrib in bh
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test ah, 80h
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jz colsub0
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mov ax, 0701h ;Scroll col down 1
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int 10h
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;Now insert a new random char at top
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mov dx, si ;Return col number into dl
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xor dh, dh ;Row 0
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xor bh, bh ;Page 0
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mov ah, 02h ;Set cursor pos
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int 10h
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mov bl, 0FFh ;Max ASCII char value
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call rndgen ;Get the char
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mov cx, 1 ;1 char to print
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mov al, bl ;Move char code into al
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mov bh, 00h ;Page number
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mov ah, 0Ah ;Write char function
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int 10h
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jmp short colexit
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colsub0:
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mov ax, 0701h ;Scroll col down 1
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int 10h
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colexit:
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pop si
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pop bx
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pop cx
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pop dx
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ret
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COL ENDP
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regen: db 4000 dup(?) ;Regen space
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;Jettison the code below here
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TRANS PROC NEAR ;Transient loader
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mov ax, 3508h ;Int 08h, timer hook, to look for signature
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int 21h
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push cs
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pop ds ;Fix ds to current cs
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cmp word ptr es:[bx+2], 55AAh ;Check signature
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jne proceed
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mov dx, OFFSET instmsg
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mov ah, 09h
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int 21h
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mov ax, 4c00h
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int 21h
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instmsg: db 'Screensaver already installed!',0Ah,0Dh, '$'
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proceed:
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;Initialise variables
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mov word ptr [ticks], 0
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mov byte ptr [inFlag], 0
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;Turn off interrupts, install handlers
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cli
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mov ax, 3508h
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int 21h ;Save old values
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mov word ptr [irq0o], bx
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mov bx, es
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mov word ptr [irq0s], bx
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mov ax,3509h
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int 21h
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mov word ptr [irq1o], bx
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mov bx, es
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mov word ptr [irq1s], bx
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mov dx, OFFSET IRQ0
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mov ax, 2508h ;ah=25 setvector, al=Int 08h
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int 21h
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push cs ;Reset ds incase int21h clobbers it
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pop ds
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mov dx, OFFSET IRQ1
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mov ax, 2509h
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int 21h
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sti ;Restart Interrupts
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;Print install message
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push cs
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pop ds
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mov dx, OFFSET sucmsg
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mov ah, 09h
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int 21h ;Print string
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;Compute size for TSR
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mov ax, OFFSET TRANS
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mov cl, 4
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shr ax, cl ;Turn number of bytes into paragraphs
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inc ax ;Add one for good measure
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mov dx, ax
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mov ax, 3100h
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int 21h
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sucmsg: db "Successfully connected to the matrix!",0Ah,0Dh,"$"
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TRANS ENDP
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CODE ENDS
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END EP

‎SC.COM

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