Posted By
Csabo
on 2009-01-11
10:55:18
|  Re: Defining a custom charset from BASIC
The round of applause goes to gerliczer, he sent me a working proof of concept of his idea, which works perfectly! I simply changed the line breaks slightly, so the whole routine fits on one Plus/4 screen (and therefore can be copy/pasted  )
10 REM SAVING ZERO PAGE VARIABLES
11 C0=PEEK(192):C1=PEEK(193)
12 C8=PEEK(200):C9=PEEK(201)
13 A9=PEEK(169):AA=PEEK(170)
14 EA=PEEK(234):EB=PEEK(235)
15 REM SAVING ADDITIONAL VARIABLE
16 T=PEEK(2024)
17 REM SETTING UP ZERO PAGE ADDRESSES
18 REM $C0-$C1 SRC1 $C8-$C9 DEST1
19 REM $A9-$AA SRC2 $EA-$EB DEST2
20 POKE192,0 : POKE193,13*16
21 POKE200,0 : POKE201,3*16+8
22 POKE169,128 : POKE170,13*16
23 POKE234,128 : POKE235,3*16+8
24 REM SETTING UP ADDITIONAL VARIABLE
25 POKE2024,128
26 REM COPYING 256 BYTES FROM CHAR-GEN
27 SYSDEC("DA4F")
28 REM RESTORING ALL SAVED VALUES
29 POKE192,C0 : POKE193,C1
30 POKE200,C8 : POKE201,C9
31 POKE169,A9 : POKE170,AA
32 POKE234,EA : POKE235,EB
33 POKE2024,T
This copies 256 bytes from $D000 to $3800 very quickly. The routine (which I'm curious as to how he found it) copies two vectors, normally that would be character and color memory. I guess that means in one cycle we could copy two 256 byte pages. To copy the entire charset would need a simple loop. Other room for optimization would be to check if we can get away with not saving some of the zeropage variables, but this method is the holy grail 
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