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Chapter 29
Saving Screens and Other VGA Mysteries

Useful Nuggets from the VGA Zen File

There are a number of VGA graphics topics that aren’t quite involved enough to warrant their own chapters, yet still cause a fair amount of programmer headscratching—and thus deserve treatment somewhere in this book. This is the place, and during the course of this chapter we’ll touch on saving and restoring 16-color EGA and VGA screens, the 16-out-of-64 colors issue, and techniques involved in reading and writing VGA control registers.

That’s a lot of ground to cover, so let’s get started!

Saving and Restoring EGA and VGA Screens

The memory architectures of EGAs and VGAs are similar enough to treat both together in this regard. The basic principle for saving EGA and VGA 16-color graphics screens is astonishingly simple: Write each plane to disk separately. Let’s take a look at how this works in the EGA’s hi-res mode 10H, which provides 16 colors at 640×350.

All we need do is enable reads from plane 0 and write the 28,000 bytes of plane 0 that are displayed in mode 10H to disk, then enable reads from plane 1 and write the displayed portion of that plane to disk, and so on for planes 2 and 3. The result is a file that’s 112,000 (28,000 * 4) bytes long, with the planes stored as four distinct 28,000-byte blocks, as shown in Figure 29.1.

The program shown later on in Listing 29.1 does just what I’ve described here, putting the screen into mode 10H, putting up some bittext so there is something to save, and creating the 112K file SNAPSHOT.SCR, which contains the visible portion of the mode 10H frame buffer.

Figure 29.1
  Saving EGA/VGA display memory.

The only part of Listing 29.1 that’s even remotely tricky is the use of the Read Map register (Graphics Controller register 4) to make each of the four planes of display memory readable in turn. The same code is used to write 28,000 bytes of display memory to disk four times, and 28,000 bytes of memory starting at A000:0000 are written to disk each time; however, a different plane is read each time, thanks to the changing setting of the Read Map register. (If this is unclear, refer back to Figure 29.1; you may also want to reread Chapter 28 to brush up on the operation of the Read Map register in particular and reading EGA and VGA memory in general.)

Of course, we’ll want the ability to restore what we’ve saved, and Listing 29.2 does this. Listing 29.2 reverses the action of Listing 29.1, selecting mode 10H and then loading 28,000 bytes from SNAPSHOT.SCR into each plane of display memory. The Map Mask register (Sequence Controller register 2) is used to select the plane to be written to. If your computer is slow enough, you can see the colors of the text change as each plane is loaded when Listing 29.2 runs. Note that Listing 29.2 does not itself draw any text, but rather simply loads the bit map saved by Listing 29.1 back into the mode 10H frame buffer.

LISTING 29.1 L29-1.ASM

; Program to put up a mode 10h EGA graphics screen, then save it
; to the file SNAPSHOT.SCR.
VGA_SEGMENT                 equ   0a000h
GC_INDEX                    equ   3ceh             ;Graphics Controller Index register
READ_MAP                    equ   4                ;Read Map register index in GC
DISPLAYED_SCREEN_SIZE       equ  (640/8)*350       ;# of displayed bytes per plane in a
                                                   ; hi-res graphics screen
stack      segment para stack ‘STACK’
                 db                  512 dup (?)
stack      ends
Data       segment     word ‘DATA’
SampleText       db    ‘This is bit-mapped text, drawn in hi-res ’
                 db    ‘EGA graphics mode 10h.’, 0dh, 0ah, 0ah
                 db    ‘Saving the screen (including this text)...’
                 db    0dh, 0ah, ‘$’
Filename         db    ‘SNAPSHOT.SCR’,0   ;name of file we’re saving to
ErrMsg1          db    ‘*** Couldn’t open SNAPSHOT.SCR ***’,0dh,0ah,‘$’
ErrMsg2          db    ‘*** Error writing to SNAPSHOT.SCR ***’,0dh,0ah,‘$’
WaitKeyMsg       db    0dh, 0ah, ‘Done. Press any key to end...’,0dh,0ah,‘$’
Handle           dw    ?                           ;handle of file we’re saving to
Plane            db    ?                           ;plane being read
Data  ends
Code           segment
               assume   cs:Code, ds:Data
Start          proc     near
               mov      ax,Data
               mov      ds,ax
; Go to hi-res graphics mode.
               mov      ax,10h     ;AH = 0 means mode set, AL = 10h selects
                                   ; hi-res graphics mode
               int      10h        ;BIOS video interrupt
; Put up some text, so the screen isn’t empty.
               mov      ah,9       ;DOS print string function
               mov      dx,offset SampleText
               int      21h
; Delete SNAPSHOT.SCR if it exists.
               mov      ah,41h     ;DOS unlink file function
               mov      dx,offset Filename
               int      21h
; Create the file SNAPSHOT.SCR.
               mov      ah,3ch        ;DOS create file function
               mov      dx,offset Filename
               sub      cx,cx         ;make it a normal file
               int      21h
               mov      [Handle],ax   ;save the handle
               jnc      SaveTheScreen ;we’re ready to save if no error
               mov      ah,9          ;DOS print string function
               mov      dx,offset ErrMsg1
               int      21h           ;notify of the error
               jmp      short Done    ;and done
; Loop through the 4 planes, making each readable in turn and
; writing it to disk. Note that all 4 planes are readable at
; A000:0000; the Read Map register selects which plane is readable
; at any one time.
               mov      [Plane],0;start with plane 0
               mov      dx,GC_INDEX
               mov      al,READ_MAP;set GC Index to Read Map register
               out      dx,al
               inc      dx
               mov      al,[Plane]          ;get the # of the plane we want
                                            ; to save
               out      dx,al               ;set to read from the desired plane
               mov      ah,40h              ;DOS write to file function
               mov      bx,[Handle]
               mov      cx,DISPLAYED_SCREEN_SIZE ;# of bytes to save
               sub      dx,dx               ;write all displayed bytes at A000:0000
               push     ds
               mov      si,VGA_SEGMENT
               mov      ds,si
               int      21h                 ;write the displayed portion of this plane
               pop      ds
               cmp      ax,DISPLAYED_SCREEN_SIZE ;did all bytes get written?
               jz       SaveLoopBottom
               mov      ah,9                ;DOS print string function
               mov      dx,offset ErrMsg2
               int      21h                 ;notify about the error
               jmp      short DoClose       ;and done
               mov      al,[Plane]
               inc      ax                  ;point to the next plane
               mov      [Plane],al
               cmp      al,3                ;have we done all planes?
               jbe      SaveLoop            ;no, so do the next plane
               mov      ah,3eh              ;DOS close file function
               mov      bx,[Handle]
               int      21h
; Wait for a keypress.
               mov      ah,9                ;DOS print string function
               mov      dx,offset WaitKeyMsg
               int      21h                 ;prompt
               mov      ah,8                ;DOS input without echo function
               int      21h
; Restore text mode.
               mov      ax,3
               int      10h
; Done.
               mov      ah,4ch;DOS terminate function
               int      21h
Start          endp
Code           ends
               end      Start

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Graphics Programming Black Book © 2001 Michael Abrash