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applesoft/spc/README.md
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@ -16,7 +16,7 @@ Let's see something weird ...
<img src="spc1.png" align="left" width=200px>At the Applesoft prompt, type `FLASH`.
Then `PRINT SPC(10)`. You should now see 10 flashing space characters.
Now, press `CTRL-BREAK`. This exits the "flash" mode (do no type `NORMAL` !!).
Now, press `CTRL-RESET`. This exits the "flash" mode (do no type `NORMAL` !!).
Type `PRINT SPC(10)` again. And ...
WOW ! WHAT IS THAT ??
@ -55,16 +55,16 @@ 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-RESET` 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 `$F3 `has an impact since the `ORA` is called whatever the display mode is.
So, before any character is displayed on screen by Applesoft, two masking operation occur on the ASCII value of the character.
So, before any character is displayed on screen by Applesoft, two masking operations occur on the ASCII value of the character.
1. an `ORA` with the value in `$F3`
2. an `AND` with the value in `$32`
`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.
`CTRL-RESET` resets 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-RESET` 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`.
@ -102,7 +102,7 @@ Remember that what happens behind the hood is that the space character (`$20`) i
<img src="spc4.png" align="left" width=200px>In fact it will not work with ASCII characters from 64 to 95, those are the uppercase letters, the `@` sign, the square brackets `][`, the backslash `\` , the caret `^` and the underscore `_` .
But all is not lost since we also have an AND mask to apply ! For now, we assumed its value was `$FF` (`255`), meaning it has no effect. But what if ...
But all is not lost since we also have an `AND` mask to apply ! For now, we assumed its value was `$FF` (`255`), meaning it has no effect. But what if ...
`$20 ORA $41` ("A" uppercase character) equals `$61` but if we do `$61 AND $DF` (thus clearing bit 5), we have `$41` back !