Apple II/II+ text/lores floating bus bug

[ryandesign]

Bob Bishop's 1982 article Have an Apple Split presents a program (in listings 1 and 2) that displays text on the top half of the screen and lores graphics on the bottom half. To run it, create a new Apple II+ in Clock Signal, press F12 to reset and get the Applesoft BASIC prompt, and paste this in:

0  REM HAVE AN APPLE SPLIT
1  REM BY BOB BISHOP
2  REM SOFTALK, OCTOBER 1982
3  FOR I = 768 TO 798: READ V: POKE I,V: NEXT
4  DATA 141,82,192,169,224,162,4,205,81,192,208,249,202,208,248,169,160,162,4,205,80,192,208,249,202,208,248,141,81,192,96
100  HOME
200  FOR K = 0 TO 39
210  POKE 1448 +K,14 *16
220  POKE 2000 +K,10 *16
230  COLOR= K +4
240  VLIN 25,45 AT K
250  NEXT K
300  VTAB 6: HTAB 17
310  PRINT "APPLE II"
400  CALL 768
500  GOTO 400

Run it with:

RUN

(This is Bishop's listing 1 with lines numbered less than 100 added by me in order to incorporate the machine language program from listing 2 and make it self-contained.)

This program does not work in Clock Signal 2023-09-10 when emulating an Apple II or II+:

(The distortion of the display starting where it switches from text to graphics flickers much more quickly than shown in this animated gif and is a separate issue.)

This program also does not work when emulating an Apple IIe, and that is not a bug. The program does not work on a real Apple IIe either.

The reason is that when an Apple II or II+ is in text or lores graphics mode, its video scanner scans memory addresses during HBL that are $1000 higher than those scanned on an Apple IIe or later. See Understanding the Apple II by Jim Sather, chapter 5.

After fixing this bug (for which I will submit a pull request), the "Have an Apple Split" program works correctly when emulating a II or II+: