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James Gregory
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29-01.md
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29-01.md
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@ -8,32 +8,32 @@ Chapter 29\
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### Useful Nuggets from the VGA Zen File {#Heading2}
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There are a number of VGA graphics topics that aren’t quite involved
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There are a number of VGA graphics topics that aren't quite involved
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enough to warrant their own chapters, yet still cause a fair amount of
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programmer headscratching—and thus deserve treatment somewhere in this
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book. This is the place, and during the course of this chapter we’ll
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book. This is the place, and during the course of this chapter we'll
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touch on saving and restoring 16-color EGA and VGA screens, the
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16-out-of-64 colors issue, and techniques involved in reading and
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writing VGA control registers.
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That’s a lot of ground to cover, so let’s get started!
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That's a lot of ground to cover, so let's get started!
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### Saving and Restoring EGA and VGA Screens {#Heading3}
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The memory architectures of EGAs and VGAs are similar enough to treat
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both together in this regard. The basic principle for saving EGA and VGA
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16-color graphics screens is astonishingly simple: Write each plane to
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disk separately. Let’s take a look at how this works in the EGA’s hi-res
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disk separately. Let's take a look at how this works in the EGA's hi-res
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mode 10H, which provides 16 colors at 640x350.
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All we need do is enable reads from plane 0 and write the 28,000 bytes
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of plane 0 that are displayed in mode 10H to disk, then enable reads
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from plane 1 and write the displayed portion of that plane to disk, and
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so on for planes 2 and 3. The result is a file that’s 112,000 (28,000 \*
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so on for planes 2 and 3. The result is a file that's 112,000 (28,000 \*
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4) bytes long, with the planes stored as four distinct 28,000-byte
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blocks, as shown in Figure 29.1.
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The program shown later on in Listing 29.1 does just what I’ve described
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The program shown later on in Listing 29.1 does just what I've described
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here, putting the screen into mode 10H, putting up some bittext so there
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is something to save, and creating the 112K file SNAPSHOT.SCR, which
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contains the visible portion of the mode 10H frame buffer.
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@ -41,7 +41,7 @@ contains the visible portion of the mode 10H frame buffer.
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\
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**Figure 29.1** *Saving EGA/VGA display memory.*
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The only part of Listing 29.1 that’s even remotely tricky is the use of
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The only part of Listing 29.1 that's even remotely tricky is the use of
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the Read Map register (Graphics Controller register 4) to make each of
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the four planes of display memory readable in turn. The same code is
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used to write 28,000 bytes of display memory to disk four times, and
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@ -52,7 +52,7 @@ back to Figure 29.1; you may also want to reread Chapter 28 to brush up
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on the operation of the Read Map register in particular and reading EGA
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and VGA memory in general.)
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Of course, we’ll want the ability to restore what we’ve saved, and
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Of course, we'll want the ability to restore what we've saved, and
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Listing 29.2 does this. Listing 29.2 reverses the action of Listing
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29.1, selecting mode 10H and then loading 28,000 bytes from SNAPSHOT.SCR
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into each plane of display memory. The Map Mask register (Sequence
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@ -73,20 +73,20 @@ by Listing 29.1 back into the mode 10H frame buffer.
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DISPLAYED_SCREEN_SIZE equ (640/8)*350 ;# of displayed bytes per plane in a
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; hi-res graphics screen
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;
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stack segment para stack ‘STACK’
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stack segment para stack ‘STACK'
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db 512 dup (?)
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stack ends
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;
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Data segment word ‘DATA’
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SampleText db ‘This is bit-mapped text, drawn in hi-res ’
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db ‘EGA graphics mode 10h.’, 0dh, 0ah, 0ah
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db ‘Saving the screen (including this text)...’
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db 0dh, 0ah, ‘$’
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Filename db ‘SNAPSHOT.SCR’,0 ;name of file we’re saving to
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ErrMsg1 db ‘*** Couldn’t open SNAPSHOT.SCR ***’,0dh,0ah,‘$’
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ErrMsg2 db ‘*** Error writing to SNAPSHOT.SCR ***’,0dh,0ah,‘$’
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WaitKeyMsg db 0dh, 0ah, ‘Done. Press any key to end...’,0dh,0ah,‘$’
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Handle dw ? ;handle of file we’re saving to
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Data segment word ‘DATA'
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SampleText db ‘This is bit-mapped text, drawn in hi-res '
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db ‘EGA graphics mode 10h.', 0dh, 0ah, 0ah
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db ‘Saving the screen (including this text)...'
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db 0dh, 0ah, ‘$'
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Filename db ‘SNAPSHOT.SCR',0 ;name of file we're saving to
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ErrMsg1 db ‘*** Couldn't open SNAPSHOT.SCR ***',0dh,0ah,‘$'
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ErrMsg2 db ‘*** Error writing to SNAPSHOT.SCR ***',0dh,0ah,‘$'
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WaitKeyMsg db 0dh, 0ah, ‘Done. Press any key to end...',0dh,0ah,‘$'
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Handle dw ? ;handle of file we're saving to
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Plane db ? ;plane being read
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Data ends
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;
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@ -102,7 +102,7 @@ by Listing 29.1 back into the mode 10H frame buffer.
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; hi-res graphics mode
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int 10h ;BIOS video interrupt
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;
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; Put up some text, so the screen isn’t empty.
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; Put up some text, so the screen isn't empty.
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;
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mov ah,9 ;DOS print string function
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mov dx,offset SampleText
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@ -121,7 +121,7 @@ by Listing 29.1 back into the mode 10H frame buffer.
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sub cx,cx ;make it a normal file
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int 21h
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mov [Handle],ax ;save the handle
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jnc SaveTheScreen ;we’re ready to save if no error
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jnc SaveTheScreen ;we're ready to save if no error
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mov ah,9 ;DOS print string function
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mov dx,offset ErrMsg1
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int 21h ;notify of the error
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