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Assembly Source File | 1988-05-14 | 39.3 KB | 792 lines

.XLIST PAGE ,132 TITLE PALET2A.ASM .LIST name palet ; Program sets the EGA palette registers from parameters given on the command ; line. ; Release 2 of palette.asm by Charles Lazo III. ; Enhanced, debugged and recompiled by: ; Charles Lazo III [72210,10] and Tim Worley [71336,730] ; ; Detail of changes: ; 'E' switch now traps changes to EGA registers in all display modes ; 'T' switch added to trap changes to EGA registers in text mode only ; 'D' parameter added to return EGA registers to default values ; 'X' switch unchanged (turns off both E and T switches) ; Entire on-screen usage text rewritten. ; Current switch status is displayed each time program is called. ; Switch settings now remain in effect until changed on the command line. ; Release 2a of palette.asm by Lew Paper ; ; Detail of changes: ; 'B' switch now modifies the blink attribute from the starting ; intense background. ; 'B' toggles it from last time palette reset it. ; 'B+' turns on blinking. ; 'B-' turns on intense background colors. ; 'U' switch now modifies underlining from the starting OFF. ; 'U' toggles it from last time palette reset it, underlining at ; the bottom of a character. ; 'U+' turns on underlining at the bottom of a character. ; 'U-' turns off underlining. ; 'U<n>' underlines at row n, where n is decimal. For an 25 line ; display, 0 <= n <= 13. For a 43 line display, 0 <= n <= 8. ; 'F' switch now disables palet. ARCMASTER apparently changes the ; mode of an EGA, and palet's resetting makes it unusable. ; 'N' switch now enables palet again. Use it if you have disabled ; palet with an F switch and want to enable it with no changes. ; Changed octal palette values to decimal. ; All changes are controlled by the variable LP_Version_2A. LP_Version_2A EQU 1 ; Use Version 3 changes rt equ 0dh lf equ 0ah of equ offset farr equ dword ptr IF LP_Version_2A no_underline_row EQU 1FH ; Underline out of display range blink_on EQU 1 ; Turn blinking on blink_off EQU 0 ; Turn intense background on BIOS_seg segment at 40H org 63H addr_6845 label word ; Port to select register for ; CRT controller org 85H points label word ; Height of character matrix in lines. ; 25 rows gives points = 14 ; 43 rows gives points = 8 ; 50 rows gives points = 7 BIOS_seg ends ENDIF ; LP_Version_2A code segment assume cs:code IF LP_Version_2A assume ds:nothing, es:nothing ; This line doesn't change the ; original program. In either ; case, all memory references which ; would normally use DS have a CS ; override. ENDIF ; LP_Version_2A org 100h begin: jmp start key1 dw 0cadeh ; key values stored in code to monitor key2 dw 0d1bah ; TSR resident status IF LP_Version_2A underline_line DB no_underline_row ; Underline row start_of_EGA_settings EQU (of underline_line) length_of_EGA_settings EQU ((of old_int_10) - start_of_EGA_settings) blink_value DB blink_off ; INT 10H BL value for blinking ENDIF ; LP_Version_2A set_values db 16 dup(0ffh) ; attribute values to place in palette ; old_int_10 must immediately follow last byte of set_values since its offset ; is used later to detect overflow old_int_10 dw ?,? ; for segment and offset of old int 10h our_call db 0 ; flag that lets us only change palette set_palette proc near inc es:our_call ; lets our set palette requests go thru push bx ; save registers used push cx IF LP_Version_2A push dx ENDIF ; LP_Version_2A push si mov cx,16 ; up to 16 palette registers to set mov si,of set_values; point to new attributes next_attribute: mov bx,16 ; compute palette register for attribute sub bx,cx lodsb ; get attribute to set cmp al,0ffh ; if ff here, then don't set jne set_register loop next_attribute ; set all specified palette registers jmp short set_done ; done set_register: mov bh,al ; attribute to bh mov ax,1000h ; set palette register function int 10h ; of ROM BIOS video service loop next_attribute ; set all specified palette registers IF LP_Version_2A set_done: mov bl,blink_value ; Set blinking through BIOS mov ax,1003H int 10H push ds ; Set underlining through direct ; hardware accesses assume ds:BIOS_seg ; Get address of CRT Controller mov ax,BIOS_seg mov ds,ax mov dx,addr_6845 ; Selector port for CRT Controller mov al,14h ; Underline register location out dx,al ; Select underline register inc dx ; Data port for CRT Controller assume ds:nothing pop ds mov al,underline_line ; location of the underline. Note ; that underline off has 1fh, which ; is usually too big to show out dx,al dec es:our_call ; disallow palette register sets pop si ; restore registers pop dx ELSE ; LP_Version_2A set_done: dec es:our_call ; disallow palette register sets pop si ; restore registers ENDIF ; LP_Version_2A pop cx pop bx ret set_palette endp graphics db 0 ; =1 if graphics, =0 if text exclude db 0 ; =1 text & graphx mode changes excluded txt_exclude db 0 ; =1 text mode changes only excluded IF LP_Version_2A palet_off db 0 ; =1 disable palet entirely ENDIF ; LP_Version_2A new_int_10 proc far ; routine modifies the int10h interrupt IF LP_Version_2A cmp palet_off,1 ; Is palet disabled? je go_on ; Yes. Bypass it. ENDIF ; LP_Version_2A cmp ah,0 ; a mode set request? je mode_set ; yeah, check it out cmp ah,11h ; SPECTRUM makes this choice; it resets ; the EGA environment je reset ; reset palette registers after the call cmp ah,12h ; CodeView uses this call je reset cmp ah,10h ; is it a set palette registers call? je func_10 ; yes, must pass our filter go_on: jmp farr old_int_10 ; pass execution to old routine mode_set: cmp al,3 ; an attempt to set a text mode? jbe text_mode ; yeah, set the mode, then reset palette cmp al,7 ; text mode 7? je text_mode ; same mov graphics,1 ; note the setting of a graphics mode cmp txt_exclude,1 ; allow graphics mode palette changes? je go_on ; yes, allow to pass without reset jmp short reset ; else, reset palette after mode set text_mode: mov graphics,0 ; note the setting of a text mode reset: pushf ; simulate an interrupt call farr old_int_10 ; and set requested text mode sti ; interrupts ok push ax ; our data is in current cs push ds push es mov ax,cs mov ds,ax mov es,ax call set_palette ; reset the palette in case set by above pop es pop ds pop ax iret ; return to our caller func_10: cmp txt_exclude,1 ; is text exclude variable set? jne no_txt_exclude ; no, not considerable cmp graphics,1 ; well, then is it a graphics mode? je go_on ; yes, allow palette changes to pass no_txt_exclude: cmp exclude,1 ; exclude other palette changes? jne go_on ; no, let the palette change go through cmp our_call,1 ; if it is our call let it go through, je go_on iret ; else do nothing new_int_10 endp ; End of resident portion of palet error: mov ah,9 ; let DOS send it int 21h mov ax,4c01h ; terminate with errorlevel 1 int 21h wrong_DOS db rt,lf,'Wrong DOS version; must use DOS 2.0 or higher.' db rt,lf,'$' no_EGA db rt,lf,'You must have an EGA installed to use this ' db 'program.',rt,lf,'$' IF LP_Version_2A not_installed db rt,lf,'palet must be installed before you use the ' db '"F" or "N" command',rt,lf,'$' ENDIF ; LP_Version_2A ; This routine is used to detect the prior load of the program as a TSR. If ; there has been no prior load, then the routine will return with the zero flag ; set. ZF will be reset if there has been a prior load and in that case the ; ES register will point to the PSP of the prior load. TSR_detect proc near ; proc will detect a prior load as a TSR mov cx,cs ; start at PSP and look at memory below dec cx ; don't hang on this PSP mov ax,20cdh ; "backwords" int 20h instruction mov bx,key1 ; first key to test mov dx,key2 ; second key to test next_paragraph: mov es,cx ; address candidate PSP cmp es:[0],ax ; make sure a PSP not some DOS buffer je int_20_found ; found int 20h signature loop next_paragraph ; continue till all paragraphs checked jmp short no_load ; no prior load; indicate and return int_20_found: cmp es:key1,bx ; match our first key? je key1_found ; found key1; maybe key2 too? loop next_paragraph ; continue till all paragraphs checked jmp short no_load ; no prior load; indicate and return key1_found: cmp es:key2,dx ; match our second key? je prior_load ; we're already installed; exit loop next_paragraph ; continue till all paragraphs checked no_load: xor ax,ax ; set zero flag to indicate not now TSR ret prior_load: or ax,ax ; reset zero flag to show prior load ret TSR_detect endp set_msg db rt,lf,'Switch set is $' x_msg db 'X',rt,lf,'$' t_msg db 'T',rt,lf,'$' e_msg db 'E',rt,lf,'$' IF LP_Version_2A f_msg db 'F',rt,lf,'$' ENDIF ; LP_Version_2A tell_switch proc near ; informs user which switches are set mov dx,of set_msg ; address switches set message mov ah,9 ; send it with DOS function 9 int 21h IF LP_Version_2A mov dx,of f_msg ; prepare 'F' switch set message cmp es:palet_off,1 ; is palet_off flag set? jz tell_set ; yes, send 'F' switch set message ENDIF ; LP_Version_2A mov dx,of x_msg ; prepare 'X' switch set message cmp es:exclude,1 ; is exclude flag set? jne tell_set ; no, send 'X' switch set message mov dx,of t_msg ; prepare 'T' set message cmp es:txt_exclude,1; is 'T' switch set? je tell_set ; yes, say 'T' set mov dx,of e_msg ; else, it's just 'E' switch set tell_set: mov ah,9 ; send it with DOS function 9 int 21h ret tell_switch endp syntax_msg db rt,lf IF LP_Version_2A db 'Usage: palet1 [D] d1 d2 ... d16 [B[+|-]][U[+|-|<n>]][T][E][X]',rt,lf db 'Where d1 through d16 are 16 two-digit decimal numbers.',rt,lf ELSE db 'Usage: palet1 [D] o1 o2 ... o16 [T][E][X]',rt,lf db rt,lf db 'Where o1 through o16 are 16 two-digit octal numbers.',rt,lf ENDIF ; LP_Version_2A db 'A period leaves that register unchanged.',rt,lf db 'Optional switches:',rt,lf db ' [D] : Returns all registers to a default value.',rt,lf db ' [T] : Traps EGA register changes in text mode only.',rt,lf db ' [E] : Traps EGA register changes in all display modes.',rt,lf db ' [X] : Turns off T & E switches (allows EGA register changes).',rt,lf IF LP_Version_2A DB ' [F] : Disable (turn oFf) palet.',rt,lf DB ' [N] : Enable (turn oN) palet.',rt,lf DB ' [B] : Blinking control, default intense background.',rt,lf DB ' [B+] turns on blinking',rt,lf DB ' [B-] turns on intense background',rt,lf DB ' [B] is equivalent to [B+] if intense and to [B-] if ' DB 'blinking',rt,lf DB ' [U] : Underlining control, default no underlining.',rt,lf DB ' [U+] turns on underlining at bottom of character',rt,lf DB ' [U<n>] turns on underlining at row n (decimal)',rt,lf DB ' [U-] turns off underlining',rt,lf DB ' [U] is equivalent to [U+] if underlining is off and to [U-] ' DB 'if on',rt,lf ENDIF ; LP_Version_2A db 'Switches T, E & X remain set unless changed on the command line.',rt,lf db 'See docs for greater detail.',rt,lf IF LP_Version_2A DB '$' ELSE db 'Palet is a public domain program.',rt,lf db rt,lf,'$' ENDIF ; LP_Version_2A set_switch db 0 ; command line requests palette set? (=0 if no) ; (not to be confused with switch_set variable) switch_set db 0 ; note any switch sets, 1=set ; (not to be confused with set_switch variable) IF LP_Version_2A decimal_number proc near ; Read a 1 or 2 digit decimal number ; from [SI] (the command line) to AL sub al,'0' ; To binary cmp byte ptr [SI],'0' ; Check for second digit jb decimal_number_exit cmp byte ptr [SI],'9' ja decimal_number_exit shl al,1 ; 10 * AL => AH mov ah,al shl al,1 shl al,1 add ah,al lodsb ; second digit => AL ; Increment SI sub al,'0' ; To binary add al,ah ; Number => AL decimal_number_exit: ret decimal_number endp ELSE ; LP_Version_2A digit_cnt db 0 ; number of parsed octal digits store_num proc near ; put octal attribute into set_values cmp digit_cnt,0 ; have we any digits to store? je no_none ; no, not a one cmp digit_cnt,1 ; is there only one digit? jne two_digits ; no, convert two digit octal to hex mov al,bh ; the one and only digit is in bh jmp short try_store ; store attribute if able two_digits: mov cl,3 ; a multiply by 8 shl bh,cl ; for the eights digit add bh,bl ; add in units digit mov al,bh ; store to set_values try_store: cmp di,of old_int_10; if 16 attributes stored, then no more jne store_it ret ; (any given after 16 are ignored) store_it: inc set_switch ; indicate a palette reg is to be set stosb xor bx,bx ; reinitialize mov digit_cnt,0 no_none: ret store_num endp do_digit proc near ; make octal digit eights or units cmp digit_cnt,0 ; have we a prior digit? jne units ; yes, have done eights so do units mov bh,al ; set eights digit into bh inc digit_cnt ; signify eights digit obtained ret units: mov bl,al ; units into bl inc digit_cnt call store_num ; got two digits, so store octal attrib ret do_digit endp ENDIF ; LP_Version_2A IF LP_Version_2A resident_ES DW 0 ; 0 also means palet is not yet ; resident org 0A00H ; To give assured debug locations ; for default values, no matter what ; the future changes are: ; Default underline line - 0a00h ; Default blink value - 0a01h ; Default palette values - ; 0a02h thru 0a11h start_of_default_settings EQU (of default_underline_line) default_underline_line DB no_underline_row ; Start by assuming no underlining default_blink_value DB blink_off ; Start by assuming intense background ENDIF ; LP_Version_2A deflt_vals db 0 ; default values for EGA registers db 1 ; 0 through 15, change these values db 2 ; to customize the default values to db 3 ; your preference db 4 ; any value of 0 through 63 is valid db 5 ; this table as it comes has the db 20 ; default (on power-up) values for db 7 ; a Tandy 3000HL with their EGA db 56 ; board configured for the EGM-1 db 57 ; monitor, yours may be different db 58 ; see docs for debug patching db 59 ; instructions db 60 ; db 61 ; db 62 ; db 63 ; end of default table IF LP_Version_2A ; Error exit for "F" and "N" commands if palet is not installed CHECK_INSTALLED MACRO LOCAL is_installed cmp resident_ES,0 ;; Is palet installed jne is_installed ;; Yes mov dx,of not_installed ;; Tell user it must be installed jmp error is_installed: ENDM ; CHECK_INSTALLED ENDIF ; LP_Version_2A start: mov ah,30h ; find DOS version int 21h cmp al,2 ; must be version 2.0 or higher jae version_ok mov dx,of wrong_DOS ; tell user this is wrong DOS version jmp error version_ok: mov ah,12h ; EGA alternate select mov bl,10h ; get EGA information mov bh,0ffh ; shall return changed if EGA present int 10h ; EGA ROM BIOS cmp bh,1 ; 0 for color, 1 for monochrome jbe EGA_here ; test concludes EGA is present mov dx,of no_EGA ; tell that EGA is not present jmp error EGA_here: mov ah,15 ; get video mode int 10h ; with call to video interrupt cmp al,3 ; text modes 0 - 3? jbe text ; yes, continue cmp al,7 ; monochrome text mode? je text ; yes, continue mov graphics,1 ; indicate a graphics mode is in effect IF LP_Version_2A text: call TSR_detect ; Detect presence of prior load as TSR jz start_parse ; Not here mov resident_es,es ; Save segment of prior load mov ax,word ptr es:underline_line ; Resident values of underline_line ; and blink_value mov word ptr underline_line,ax ; Non-resident values of underline_line ; and blink_value match resident start_parse: mov ax,cs ; Reestablish es as non-resident ; segment mov es,ax mov si,80h ; reference command line parameters ELSE text: mov si,80h ; reference command line parameters ENDIF ; LP_Version_2A mov di,of set_values; address store for palette values IFE LP_Version_2A xor bx,bx ; octal attribute accumulated in bx ENDIF ; IFE LP_Version_2A cld lodsb ; get command character count cmp al,1 ; anything on command line? jae next_value ; yes, decode it IF LP_Version_2A cmp resident_ES,0 ; has the program been loaded as a TSR? ELSE call TSR_detect ; has the program been loaded as a TSR? ENDIF ; LP_Version_2A jz just_syntax ; no, just output syntax message IF LP_Version_2A mov es,resident_ES ; Segment of resident version ENDIF ; LP_Version_2A call tell_switch ; report switch state just_syntax: mov dx,of syntax_msg; address syntax message jmp error next_value: lodsb ; get next entry from command line cmp al,rt ; end of command? jne an_E? ; is it the exclude command? IFE LP_Version_2A call store_num ; store octal num in bh:bl as attribute ENDIF ; IFE LP_Version_2A jmp parse_done an_E?: mov cl,al ; process in cl and cl,0dfh ; capitalize if letter cmp cl,'E' ; is it an "E"? IF LP_Version_2A jne an_F? ; no, is it a "F"? ELSE ; LP_Version_2A jne a_D? ; no, is it a "D"? ENDIF ; LP_Version_2A mov exclude,1 ; set exclude flag: all modes trapped mov txt_exclude,0 ; text exclude turned off inc switch_set ; note that switch turned on jmp short next_value; continue till command parsed IF LP_Version_2A an_F?: cmp cl,'F' ; Is it an "F"? jne an_N? ; No. Is it an "N"? CHECK_INSTALLED ; Error exit if not installed mov palet_off,1 ; Set disable flag jmp short next_value ; continue until command parsed an_N?: cmp cl,'N' ; Is it an "N"? jne a_D? ; No. Is it a "D"? CHECK_INSTALLED ; Error exit if not installed jmp short next_value ; Since palet iniitializes palet_off, ; the disable flag, to 0, there is ; nothing to reset. Continue until ; command parsed. ENDIF ; LP_Version_2A a_D?: cmp cl,'D' ; is it a D? jne a_T? ; go on if not inc set_switch ; indicate a palette reg change push si ; save critical registers push di IF LP_Version_2A mov cx,length_of_EGA_settings mov di,start_of_EGA_settings ; Values to actually use mov si,start_of_default_settings ELSE mov cx,16 ; number of palette registers mov di,of set_values; point to reg values buffer area mov si,of deflt_vals; load address of default table ENDIF ; LP_Version_2A rep movsb ; copy defaults to be set later pop di ; restore registers pop si ; jmp short next_value; get next parameter (if any) a_T?: cmp cl,'T' ; is it a "T"? jne an_X? ; no, check for an "X" mov exclude,1 ; set the exclude flag mov txt_exclude,1 ; set the text flag, only txt trapped inc switch_set ; note that switch set jmp next_value ; continue till command parsed an_X?: cmp cl,'X' ; is it an "X"? IF LP_Version_2A jne a_B? ; no, check for a "B" ELSE jne a_period? ; no, is it a period? ENDIF ; IF LP_Version_2A mov exclude,0 ; reset exclude flag: others can set mov txt_exclude,0 ; reset text exclude flag inc switch_set ; note that switch set jmp next_value ; continue till command parsed IF LP_Version_2A a_B?: cmp cl, 'B' ; Is it a "B"? jne a_U? ; No, check for a "U" inc set_switch ; indicate a palette reg is to be set cmp byte ptr [si],'+' ; Set blinking on? je request_blinking_on ; Yes cmp byte ptr [si],'-' ; Set blinking off je request_blinking_off ; Yes cmp blink_value,blink_on ; Must want to switch jz request_blinking_off_1 jmp short request_blinking_on_1 request_blinking_on: inc si ; Pass "+" on command line request_blinking_on_1: mov blink_value, blink_on jmp next_value ; Get next parameter (if any) request_blinking_off: inc si ; Pass "-" on command line request_blinking_off_1: mov blink_value, blink_off jmp next_value ; Get next parameter (if any) a_U?: cmp cl, 'U' ; Is it a "U"? jne a_period? ; no, is it a period? inc set_switch ; indicate a palette reg is to be set cmp byte ptr [si],'+' ; Set underlining on? je request_underlining_on ; Yes cmp byte ptr [si],'-' ; Set underlining off je request_underlining_off ; Yes cmp byte ptr [si],'0' ; Check for decimal digit to start ; underline row jb underline_switch_request cmp byte ptr [si],'9' ja underline_switch_request lodsb ; First digit => AL ; Increment SI call decimal_number ; Set the underline row in AL ; Point SI at the following ; character mov underline_line,al jmp next_value ; Get next parameter (if any) underline_switch_request: cmp es:underline_line,no_underline_row jz request_underlining_on_1 jmp short request_underlining_off_1 request_underlining_on: inc si ; Pass "+" on command line request_underlining_on_1: push ds assume ds:BIOS_seg mov ax,BIOS_seg mov ds,ax mov ax,points ; Number of lines in a character assume ds:nothing pop ds dec ax ; Line number of last line mov underline_line, al ; Underline at last line jmp next_value ; Get next parameter (if any) request_underlining_off: inc si ; Pass "-" on command line request_underlining_off_1: mov underline_line, no_underline_row jmp next_value ; Get next parameter (if any) ENDIF ; IF LP_Version_2A a_period?: cmp al,'.' ; a period? jne a_space? ; no, should be whitespace or octal num IFE LP_Version_2A call store_num ; store octal num in bh:bl as attribute ENDIF ; IFE LP_Version_2A inc di ; a period says we skip one jmp next_value ; continue till command parsed a_space?: cmp al,' ' ; a space? jne a_tab? ; no, is it a tab? IFE LP_Version_2A call store_num ; store octal num in bh:bl as attribute ENDIF ; IFE LP_Version_2A jmp next_value ; continue till command parsed a_tab?: cmp al,9 ; a tab? jne a_digit? ; no, an octal digit? IFE LP_Version_2A call store_num ; store octal num in bh:bl as attribute ENDIF ; IFE LP_Version_2A jmp next_value ; continue till command parsed a_digit?: cmp al,'0' ; is it ASCII zero or greater? IF LP_Version_2A jae decimal_digit? ELSE jae octal_digit? ; yes, test if octal digit ENDIF ; IF LP_Version_2A mov dx,of syntax_msg; address syntax message jmp error ; no, must be a syntax error IF LP_Version_2A decimal_digit?: cmp al,'9' ; is it 9 or less? jbe is_decimal ; yes, convert to binary ELSE octal_digit?: cmp al,'7' ; is it 7 or less? jbe is_octal ; yes, convert to binary ENDIF ; IF LP_Version_2A mov dx,of syntax_msg; address syntax message jmp error ; no, must be a syntax error IF LP_Version_2A is_decimal: call decimal_number ; Set the palette value in AL ; Point SI at the following ; character cmp di,of old_int_10; if 16 attributes stored, then no more jne store_it jmp next_value store_it: inc set_switch ; indicate a palette reg is to be set stosb ELSE is_octal: sub al,'0' ; make it binary call do_digit ; place in bh or bl or store number ENDIF ; IF LP_Version_2A jmp next_value ; continue till command parsed IF LP_Version_2A parse_done: cmp resident_ES,0 ; detect presence of prior load as TSR ELSE parse_done: call TSR_detect ; detect presence of prior load as TSR ENDIF ; LP_Version_2A jz not_here ; ZF set on return => no prior load IF LP_Version_2A mov es,resident_ES ; Segment of resident version mov al,palet_off ; Always reset disable flag mov es:palet_off,al ENDIF ; LP_Version_2A cmp set_switch,0 ; has palette registers to set changed? je no_set ; no, don't copy to resident part IF LP_Version_2A mov si,start_of_EGA_settings ; transfer new values to our resident mov di,si ; TSR routine mov cx,length_of_EGA_settings ELSE mov si,of set_values; transfer new values to our resident mov di,si ; TSR routine mov cx,16 ENDIF ;LP_Version_2A rep movsb no_set: call set_palette ; set palette registers as requested cmp switch_set,0 ; check for switch setting change je terminate ; don't change switches--none entered mov al,exclude ; get exclude flag mov es:exclude,al ; and place in TSR mov al,txt_exclude ; get text only exclude flag mov es:txt_exclude,al ; and place it in TSR terminate: call tell_switch ; tell user which switches are set mov ax,4c00h ; terminate with error level 0 int 21h IF LP_Version_2A not_here: ELSE not_here: mov ax,cs ; reset es to our data mov es,ax ENDIF ; LP_Version_2A call set_palette ; set palette registers as requested mov ax,3510h ; get current int 10h address int 21h mov old_int_10,bx ; store offset of current int 10h mov old_int_10+2,es ; store segment of current int 10h mov dx,of new_int_10; address our spliced-in routine mov ax,2510h ; tell DOS to use our int 10h routine int 21h mov dx,of error ; address first byte we can throw away add dx,0fh ; compute number of paragraphs to save shr dx,1 shr dx,1 shr dx,1 shr dx,1 mov ax,3100h ; make a TSR with error code 0 int 21h code ends end begin