bitmap-editor/applesoft/spc

Use SPC() to repeat any character !

Introduction

You know how SPC() can be used to PRINT a number of space characters. For example PRINT SPC(10) will print 10 space characters.

Why didn't they allow to print something else than space characters ? It would have been interesting (?) to have the ability to repeat a sequence of any character.

Maybe like PRINT REPT("*",10) would print 10 asterisks.

But Applesoft does not provide such an instruction. So are we doomed to use PRINT "**********"?

Here's a technique that will allow you to repeat any character, even in FLASH and INVERSE without using additional 6502 routines.

Discovery

Let's see something weird ...

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 !!). Type PRINT SPC(10) again. And ...

WOW ! WHAT IS THAT ??

What are those inverted single quote characters doing here ? "Something" has replaced space characters with those inverted single quotes ...

If you have a loaded Applesoft program, I encourage you to LIST it. If not, quickly type a short one and see the results ...

As you can see, something is messed up !

Explanation

To understand what's happening here, you need to know how characters are printed on screen by Applesoft.

The general routine to print characters on screen is in $DB5C. Here's the routine, taken from S-C documentor website

               1950 *      PRINT CHAR FROM (A)
               1960 *
               1970 *      NOTE: POKE 243,32 ($20 IN $F3) WILL CONVERT
               1980 *      OUTPUT TO LOWER CASE.  THIS CAN BE CANCELLED
               1990 *      BY NORMAL, INVERSE, OR FLASH OR POKE 243,0.
               2000 *--------------------------------
DB5C- 09 80    2010 OUTDO  ORA #$80     PRINT (A)
DB5E- C9 A0    2020        CMP #$A0     CONTROL CHR?
DB60- 90 02    2030        BCC .1       SKIP IF SO
DB62- 05 F3    2040        ORA FLASH.BIT   =$40 FOR FLASH, ELSE $00
DB64- 20 ED FD 2050 .1     JSR MON.COUT "AND"S WITH $3F (INVERSE), $7F (FLASH)
DB67- 29 7F    2060        AND #$7F
DB69- 48       2070        PHA
DB6A- A5 F1    2080        LDA SPEEDZ   COMPLEMENT OF SPEED #
DB6C- 20 A8 FC 2090        JSR MON.WAIT   SO SPEED=255 BECOMES (A)=1
DB6F- 68       2100        PLA
DB70- 60       2110        RTS

The routine is called with the accumulator containing the character to print every time Applesoft needs to print something (like when using PRINT or INPUT or ... SPC !)

The routine that will effectively print the character on screen is COUT (in $FDEDhere 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.

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 $32is 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.

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.

Taking advantage of what we know

Of course Applesoft expects and uses some specific values in $F3 and $32.

	NORMAL	FLASH	INVERSE
$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 POKEing 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.