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Z80 & 68k Assembly =>
z80 & ez80 Assembly
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Tyraniek
Member
Joined: 07 Jun 2003
Posts: 133
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 Posted: 11 Jun 2003 03:35:29 am Post subject: |
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How does it work ? I've read that to inverse all the LCD's pixels, I can use this routine (which works perfectly, and very quickly !) :
Code:
ld hl,plotsscreen
ld bc,768
inverse:
ld a, (hl)
cpl ; complement accumulator
ld (hl), a
inc hl
dec bc
ld a, b
or c
jr nz, inverse
B_CALL(_PDspGrph)
ret
What's the role of "CPL" ? And by the way, what is "_PDspGrph" exactly ? How does this routine works ?
In fact, the routine just does
Code:
ld a, (hl)
cpl ; complement accumulator
ld (hl), a
inc hl
768 times, because there is 768 bytes (96 pixels large, 64 pixels hight, 1 bit for 1 pixel, so 1 byte for 8 pixels), but what does this mysterious complement accumulator do ?
thx |
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62 52 53 53
Formerly known as 62 52 53 53
Active Member
Joined: 30 May 2003
Posts: 607
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| Posted: 11 Jun 2003 02:24:54 pm Post subject: |
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| CPL inverts the accumulator |
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NETWizz
Byte by bit
Bandwidth Hog
Joined: 20 May 2003
Posts: 2369
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| Posted: 11 Jun 2003 02:50:18 pm Post subject: |
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As he said, CPL inverts the accumulator.
%11010000
invert
%00101111
As you know, there are 768 bytes per picture, and the plotSScreen contains the picture.
What the routine does is read a byte of the plotscreen and place it in the accumulator. Once in the accumulator, it uses CPL to inverse all the bits; finally, it writes the byte back to the plotSScreen at the same location.
Please note that hl points to a ram address. ld a,(hl) reads the ram address pointed to by hl.
Now, dec BC decrements the byte counter by one to keep track of how many bytes are left to invert.
inc HL points hl to the next byte of the plotSScreen to perform the same task.
ld a,b
or c ;checks if bc=0
If BC is 0, the Z flag will be set.
If the Z flag is not set, BC is not 0; therfore, we have more bytes to invert, and the NZ flag will be set because the Z flag is not set.
Jr nz,inverse continues the inverse function if the nz flag is set (if we have more pixels to invert)
I have no idea what bcall(pdspgrph) does, but I would assume it copies the plotSScreen to the Screen, so the user can see an inverted picture.
If you are a speed freak, and need the pixels changed faster, you could modify ionfast copy to invert all the pixels. |
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Tyraniek
Member
Joined: 07 Jun 2003
Posts: 133
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| Posted: 12 Jun 2003 06:31:00 am Post subject: |
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| OK, it's an easy routine in fact ! |
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