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@ -55,9 +55,9 @@ The routine is called with the accumulator containing the character to print eve
The routine that will effectively print the character on screen is `COUT` (in `$FDED`here named `MON.COUT`) but this routine here is the pre-treatment of the character to print.
As you can see, before calling `MON.COUT`, an `ORA` with zero-page memory `$F3` is executed. This `ORA` is needed to display characters in flash mode. The problem is that `$F3`, even after a `CTRL-BREAK` is not reset and still contains `#$40` (decimal 64), meaning that Applesoft is still (partially -- see below why) in flash mode.
As you can see, before calling `MON.COUT`, an `ORA` with zero-page memory `$F3` is executed. This `ORA` is needed to display characters in flash mode. The problem is that `$F3`, even after a `CTRL-BREAK` is not reset and still contains `$40` (decimal 64), meaning that Applesoft is still (partially -- see below why) in flash mode.
But if it's in flash mode, how comes it prints NORMAL single quotes and not flashing characters ? Because `$F3` is just a mask and is not enough to flash the characters on screen. Another mask, in zero-page `$32` is also used, but this time by the `MON.COUT` routine. In fact `$32`is usually considered to be the memory that indicates if we are in normal (value `#$FF`, decimal `255`), flash (value `#$7F`, decimal `127`) or inverse (value `#$3F`, decimal `63`) modes. But for the flash mode, the mask in `$F3` is equally primordial. In fact, even in normal and inverse modes, the value in `$F3 `has an impact since the `ORA` is called whatever the display mode is.
But if it's in flash mode, how comes it prints NORMAL single quotes and not flashing characters ? Because `$F3` is just a mask and is not enough to flash the characters on screen. Another mask, in zero-page `$32` is also used, but this time by the `MON.COUT` routine. In fact `$32`is usually considered to be the memory that indicates if we are in normal (value `$FF`, decimal `255`), flash (value `$7F`, decimal `127`) or inverse (value `$3F`, decimal `63`) modes. But for the flash mode, the mask in `$F3` is equally primordial. In fact, even in normal and inverse modes, the value in `$F3 `has an impact since the `ORA` is called whatever the display mode is.
So, before any character is displayed on screen by Applesoft, two masking operation occur on the ASCII value of the character.
@ -66,11 +66,26 @@ So, before any character is displayed on screen by Applesoft, two masking operat
`CTRL-BREAK` reset the value in `$32` to `255` ("normal" display mode) but it does not touch the value in `$F3`. That's why we have these display glitches if we `CTRL-BREAK` after `FLASH`. Clearly, it's a bug.
## Taking advantage of what we know
Of course Applesoft expects and uses some specific values in `$F3` and `$32`.
| | NORMAL | FLASH | INVERSE |
|--|--|--|--|
| **$32** | 255 ($FF)| 127 ($7F) | 63 ($3F) |
| **$F3** | 0 ($00) | 64 ($40) | 0 ($00)
| **$32 (50)** | $FF (255)| $7F (127) | $3F (63) |
| **$F3 (243)** | $00 (0)| $40 (64) | $00 (0)
Now, if we play a bit with the values in those two memory locations we will alter the way Applesoft displays characters on screen.
Two well-known (useless but kind of fun) examples are
1. `POKE 243,32` to lowercase everything, notably the `LIST` command. `POKE 243,0` will return to normal display.
2. `POKE 50, 128` to sort-of disable printing. `LIST` and `CATALOG` will appear empty and the `]` prompt will disappear as well.
Notice that `POKE`ing in 243 influences only Applesoft and not the system ! That's because the `ORA $F3` only occurs within an Applesoft 6502 routine !
So ... `SPC()` is an Applesoft display routine. And as such, it will be influenced by both the values in `$32` and `$F3`. Now we just have to tweak those values to get expected results, that is, replace the space character by the character we want.
It turns out it's quite easy to do. You take the ASCII value of the character you want to display instead of the space character and you `POKE` it in `243` (`$F3`).
For example, the asterisk has an ASCII value of `42`, so we just do `POKE 243,42` and then `PRINT SPC(10)`. After that we restore the original state with `POKE 243,0`.
Now