Appendix C Identifying Video Subsystems Programs need to determine the configuration of the video hardware on which they are run for two reasons. One is to maintain portability. A program that recognizes the video subsystems in the computer in which it runs can adapt itself to specific hardware configurations. Imagine, for example, a program that displays both text and graphics images. This program could display text and graphics on a single screen in a computer with only one video subsystem, but it could also take full advantage of a dual-display configuration by placing text on one screen and graphics on the other. Another reason to enable a program to examine its video hardware environment is to allow use of the fastest possible video output routines. For example, if your program runs in an alphanumeric mode on a CGA, you may need to avoid snow by synchronizing with the CRT Controller's timing signals. However, this overhead can be avoided if the program is running on some other video subsystem. If your program "knows" that it's not running on a CGA, it can use faster video output routines that omit the overhead of avoiding snow. CGA and Clones Unfortunately, for Color Graphics Adapters and clones, no reliable way exists to determine whether the hardware manages conflicts over video buffer memory access without display interference (see Chapter 3). If your program must run on a CGA, you might wish to ask the user to configure your alphanumeric output routines by testing whether or not they produce snow. You can also detect whether your program is running on a CGA work- alike that does not have the alphanumeric snow problem. If you know that your program may run on a CGA work-alike such as the video hardware built into a COMPAQ or an AT&T 6300, you can search the ROM BIOS for a string indicating the name of the computer, for example, "COMPAQ". You might also inspect the ROM BIOS ID byte at F000:FFFE to determine whether your program is running on a member of the IBM PC family that does not have the snow problem (such as the PCjr). Other Video Adapters Although determining whether a particular CGA or clone has a problem with alphanumeric snow can be hard, distinguishing among the various common IBM video adapters is relatively easy. Some of the techniques described in this appendix rely on serendipitous peculiarities of different adapters' firmware or hardware, but all are based on IBM and Hercules recommendations. PS/2s On the PS/2s, INT 10H function 1AH lets you determine which video subsystems are present and active in the computer (see Appendix A). Of course, the PS/2 video BIOS does not recognize non-IBM video adapters. For example, if you use a Hercules adapter in a PS/2 Model 30, a call to INT 10H function 1AH returns only the information that an MDA-compatible adapter is present in the system. Identifying the adapter is then up to you. VideoID, the routine in Listing C-1, detects the presence of either one or two adapters. If two adapters are present, VideoID indicates which is active (that is, which one the BIOS is currently using for output). The techniques used to identify each adapter are described in the listing. 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Listing C-1. A routine to identify PC and PS/2 video subsystems. TITLE 'Listing C-1' NAME VideoID PAGE 55,132 ; ; Name: VideoID ; ; Function: Detects the presence of various video subsystems and associated ; monitors. ; ; Caller: Microsoft C: ; ; void VideoID(VIDstruct); ; ; struct ; { ; char VideoSubsystem; ; char Display; ; } ; *VIDstruct[2]; ; ; Subsystem ID values: ; 0 = (none) ; 1 = MDA ; 2 = CGA ; 3 = EGA ; 4 = MCGA ; 5 = VGA ; 80h = HGC ; 81h = HGC+ ; 82h = Hercules InColor ; ; Display types: 0 = (none) ; 1 = MDA-compatible monochrome ; 2 = CGA-compatible color ; 3 = EGA-compatible color ; 4 = PS/2-compatible monochrome ; 5 = PS/2-compatible color ; ; ; The values returned in VIDstruct[0].VideoSubsystem and ; VIDstruct[0].Display indicate the currently active subsystem. ; ARGpVID EQU word ptr [bp+4] ; stack frame addressing VIDstruct STRUC ; corresponds to C data structure Video0Type DB ? ; first subsystem type Display0Type DB ? ; display attached to first subsystem Video1Type DB ? ; second subsystem type Display1Type DB ? ; display attached to second subsystem VIDstruct ENDS Device0 EQU word ptr Video0Type[di] Device1 EQU word ptr Video1Type[di] MDA EQU 1 ; subsystem types CGA EQU 2 EGA EQU 3 MCGA EQU 4 VGA EQU 5 HGC EQU 80h HGCPlus EQU 81h InColor EQU 82h MDADisplay EQU 1 ; display types CGADisplay EQU 2 EGAColorDisplay EQU 3 PS2MonoDisplay EQU 4 PS2ColorDisplay EQU 5 TRUE EQU 1 FALSE EQU 0 DGROUP GROUP _DATA _TEXT SEGMENT byte public 'CODE' ASSUME cs:_TEXT,ds:DGROUP PUBLIC _VideoID _VideoID PROC near push bp ; preserve caller registers mov bp,sp push si push di ; initialize the data structure that will contain the results mov di,ARGpVID ; DS:DI -> start of data structure mov Device0,0 ; zero these variables mov Device1,0 ; look for the various subsystems using the subroutines whose addresses are ; tabulated in TestSequence; each subroutine sets flags in TestSequence ; to indicate whether subsequent subroutines need to be called mov byte ptr CGAflag,TRUE mov byte ptr EGAflag,TRUE mov byte ptr Monoflag,TRUE mov cx,NumberOfTests mov si,offset DGROUP:TestSequence L01: lodsb ; AL := flag test al,al lodsw ; AX := subroutine address jz L02 ; skip subroutine if flag is false push si push cx call ax ; call subroutine to detect subsystem pop cx pop si L02: loop L01 ; determine which subsystem is active call FindActive pop di ; restore caller registers and return pop si mov sp,bp pop bp ret _VideoID ENDP ; ; FindPS2 ; ; This subroutine uses INT 10H function 1Ah to determine the video BIOS ; Display Combination Code (DCC) for each video subsystem present. ; FindPS2 PROC near mov ax,1A00h int 10h ; call video BIOS for info cmp al,1Ah jne L13 ; exit if function not supported (i.e., ; no MCGA or VGA in system) ; convert BIOS DCCs into specific subsystems & displays mov cx,bx xor bh,bh ; BX := DCC for active subsystem or ch,ch jz L11 ; jump if only one subsystem present mov bl,ch ; BX := inactive DCC add bx,bx mov ax,[bx+offset DGROUP:DCCtable] mov Device1,ax mov bl,cl xor bh,bh ; BX := active DCC L11: add bx,bx mov ax,[bx+offset DGROUP:DCCtable] mov Device0,ax ; reset flags for subsystems that have been ruled out mov byte ptr CGAflag,FALSE mov byte ptr EGAflag,FALSE mov byte ptr Monoflag,FALSE lea bx,Video0Type[di] ; if the BIOS reported an MDA ... cmp byte ptr [bx],MDA je L12 lea bx,Video1Type[di] cmp byte ptr [bx],MDA jne L13 L12: mov word ptr [bx],0 ; ... Hercules can't be ruled out mov byte ptr Monoflag,TRUE L13: ret FindPS2 ENDP ; ; FindEGA ; ; Look for an EGA. This is done by making a call to an EGA BIOS function ; which doesn't exist in the default (MDA, CGA) BIOS. FindEGA PROC near ; Caller: AH = flags ; Returns: AH = flags ; Video0Type and ; Display0Type updated mov bl,10h ; BL := 10h (return EGA info) mov ah,12h ; AH := INT 10H function number int 10h ; call EGA BIOS for info ; if EGA BIOS is present, ; BL <> 10H ; CL = switch setting cmp bl,10h je L22 ; jump if EGA BIOS not present mov al,cl shr al,1 ; AL := switches/2 mov bx,offset DGROUP:EGADisplays xlat ; determine display type from switches mov ah,al ; AH := display type mov al,EGA ; AL := subystem type call FoundDevice cmp ah,MDADisplay je L21 ; jump if EGA has a monochrome display mov CGAflag,FALSE ; no CGA if EGA has color display jmp short L22 L21: mov Monoflag,FALSE ; EGA has a mono display, so MDA and ; Hercules are ruled out L22: ret FindEGA ENDP ; ; FindCGA ; ; This is done by looking for the CGA's 6845 CRTC at I/O port 3D4H. ; FindCGA PROC near ; Returns: VIDstruct updated mov dx,3D4h ; DX := CRTC address port call Find6845 jc L31 ; jump if not present mov al,CGA mov ah,CGADisplay call FoundDevice L31: ret FindCGA ENDP ; ; FindMono ; ; This is done by looking for the MDA's 6845 CRTC at I/O port 3B4H. If ; a 6845 is found, the subroutine distinguishes between an MDA ; and a Hercules adapter by monitoring bit 7 of the CRT Status byte. ; This bit changes on Hercules adapters but does not change on an MDA. ; ; The various Hercules adapters are identified by bits 4 through 6 of ; the CRT Status value: ; ; 000b = HGC ; 001b = HGC+ ; 101b = InColor card ; FindMono PROC near ; Returns: VIDstruct updated mov dx,3B4h ; DX := CRTC address port call Find6845 jc L44 ; jump if not present mov dl,0BAh ; DX := 3BAh (status port) in al,dx and al,80h mov ah,al ; AH := bit 7 (vertical sync on HGC) mov cx,8000h ; do this 32768 times L41: in al,dx and al,80h ; isolate bit 7 cmp ah,al loope L41 ; wait for bit 7 to change jne L42 ; if bit 7 changed, it's a Hercules mov al,MDA ; if bit 7 didn't change, it's an MDA mov ah,MDADisplay call FoundDevice jmp short L44 L42: in al,dx mov dl,al ; DL := value from status port mov ah,MDADisplay ; assume it's a monochrome display mov al,HGC ; look for an HGC and dl,01110000b ; mask off bits 4 thru 6 jz L43 mov al,HGCPlus ; look for an HGC+ cmp dl,00010000b je L43 ; jump if it's an HGC+ mov al,InColor ; it's an InColor card mov ah,EGAColorDisplay L43: call FoundDevice L44: ret FindMono ENDP ; ; Find6845 ; ; This routine detects the presence of the CRTC on a MDA, CGA or HGC. ; The technique is to write and read register 0Fh of the chip (cursor ; low). If the same value is read as written, assume the chip is ; present at the specified port addr. ; Find6845 PROC near ; Caller: DX = port addr ; Returns: cf set if not present mov al,0Fh out dx,al ; select 6845 reg 0Fh (Cursor Low) inc dx in al,dx ; AL := current Cursor Low value mov ah,al ; preserve in AH mov al,66h ; AL := arbitrary value out dx,al ; try to write to 6845 mov cx,100h L51: loop L51 ; wait for 6845 to respond in al,dx xchg ah,al ; AH := returned value ; AL := original value out dx,al ; restore original value cmp ah,66h ; test whether 6845 responded je L52 ; jump if it did (cf is reset) stc ; set carry flag if no 6845 present L52: ret Find6845 ENDP ; ; FindActive ; ; This subroutine stores the currently active device as Device0. The ; current video mode determines which subsystem is active. ; FindActive PROC near cmp word ptr Device1,0 je L63 ; exit if only one subsystem cmp Video0Type[di],4 ; exit if MCGA or VGA present jge L63 ; (INT 10H function 1AH cmp Video1Type[di],4 ; already did the work) jge L63 mov ah,0Fh int 10h ; AL := current BIOS video mode and al,7 cmp al,7 ; jump if monochrome je L61 ; (mode 7 or 0Fh) cmp Display0Type[di],MDADisplay jne L63 ; exit if Display0 is color jmp short L62 L61: cmp Display0Type[di],MDADisplay je L63 ; exit if Display0 is monochrome L62: mov ax,Device0 ; make Device0 currently active xchg ax,Device1 mov Device0,ax L63: ret FindActive ENDP ; ; FoundDevice ; ; This routine updates the list of subsystems. ; FoundDevice PROC near ; Caller: AH = display # ; AL = subsystem # ; Destroys: BX lea bx,Video0Type[di] cmp byte ptr [bx],0 je L71 ; jump if 1st subsystem lea bx,Video1Type[di] ; must be 2nd subsystem L71: mov [bx],ax ; update list entry ret FoundDevice ENDP _TEXT ENDS _DATA SEGMENT word public 'DATA' EGADisplays DB CGADisplay ; 0000b, 0001b (EGA switch values) DB EGAColorDisplay ; 0010b, 0011b DB MDADisplay ; 0100b, 0101b DB CGADisplay ; 0110b, 0111b DB EGAColorDisplay ; 1000b, 1001b DB MDADisplay ; 1010b, 1011b DCCtable DB 0,0 ; translate table for INT 10h func 1Ah DB MDA,MDADisplay DB CGA,CGADisplay DB 0,0 DB EGA,EGAColorDisplay DB EGA,MDADisplay DB 0,0 DB VGA,PS2MonoDisplay DB VGA,PS2ColorDisplay DB 0,0 DB MCGA,EGAColorDisplay DB MCGA,PS2MonoDisplay DB MCGA,PS2ColorDisplay TestSequence DB TRUE ; this list of flags and addresses DW FindPS2 ; determines the order in which this ; program looks for the various EGAflag DB ? ; subsystems DW FindEGA CGAflag DB ? DW FindCGA Monoflag DB ? DW FindMono NumberOfTests EQU ($-TestSequence)/3 _DATA ENDS END 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The VideoID routine checks for adapters by a process of elimination. For example, if the routine is run on a PS/2, the INT 10H call returns the desired information. On PC/XTs and PC/ATs, if an EGA with a monochrome display is detected, there is no reason to look for an MDA or a Hercules card in the same system. If a monochrome adapter is present, the routine differentiates between the MDA and the various Hercules adapters. �袴敲 INT 10H function 1AH on the VGA adapter fails to report � T � the presence of the MCGA when the adapter is installed in � I � a PS/2 Model 30. Also, function 1AH in the MCGA ignores � P � the presence of an EGA if one is installed in a Model 30. 훤袴� If you are concerned about these combinations, you must test for them explicitly after you call INT 10H function 1AH. (In the first situation, inspect the motherboard BIOS identfication byte at F000:FFFE to detect the presence of a Model 30. In the second situation, execute INT 10H function 12H with BL = 10H to detect the presence of an EGA.) The C program in Listing C-2 demonstrates how you might use VideoID. 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Listing C-2. Calling VideoID from a C program. /* Listing C-2 */ main() { char *SubsystemName(); char *DisplayName(); static struct { char Subsystem; char Display; } VIDstruct[2]; /* detect video subsystems */ VideoID( VIDstruct ); /* show results */ printf( "Video subsystems in this computer:" ); printf( "\n %s (%s)", SubsystemName(VIDstruct[0].Subsystem), DisplayName(VIDstruct[0].Display) ); if ( VIDstruct[1].Subsystem ) printf( "\n %s (%s)", SubsystemName(VIDstruct[1].Subsystem), DisplayName(VIDstruct[1].Display) ); } char *SubsystemName( a ) char a; { static char *IBMname[] = { "(none)", "MDA", "CGA", "EGA", "MCGA", "VGA" }; static char *Hercname[] = { "HGC", "HGC+", "InColor" }; if ( a & 0x80 ) return ( Hercname[a & 0x7F] ); else return( IBMname[a] ); } char *DisplayName( d ) char d; { static char *name[] = { "(none)", "MDA-compatible monochrome display", "CGA-compatible color display", "EGA-compatible color display", "PS/2-compatible monochrome display", "PS/2-compatible color display" }; return( name[d] ); } 컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�