[Coco] Extended Color BASIC memory

[William Astle]

That bit getting loaded into A is the upper 8 bits of the first usable 
graphics page after the Extended Basic or Disk Basic system variables. 
Thus, that code will work on a cassette system or a disk system. It will 
also work properly if someone set up some non-default environment with 
the FILES command.

Normally, $BC will have 6 in it for cassette basic and 14 for disk basic 
right after startup. The "lda" is getting the value at $BC, not $BC into 
A. Thus, the result is that Y will have $0601 or $0E01 in the typical 
case of a cold start.

The JMP transfers control back into the PCLEAR command after all the 
parameter checking at which point it sets up things correctly for basic 
to continue, relocates the program, and gets on with things.

On 13-04-03 10:41 PM, Kip Koon wrote:
> Allen,
> The 6809 machine code in the date statement disassembles as shown in the
> following listing.
> -------------------------------------
> 0000: C6 01          ldb        #01
> 0002: 96 BC          lda        0x00BC
> 0004: 1F 02           tfr         d,y
> 0006: 7E 96 A3     jmp      0x96A3
> --------------------------------------
> AccB is loaded with $01.
> I'll have to check with the variable definitions in the unraveled series to
> see what $00BC is but AccA is loaded with the contents of memory location
> $00BC.  AccD (AccA+AccB concatenated) is TFR (transferred) to Index Register
> Y.
> The JMP instruction is jumping back into ECB at address $96A3 with a
> parameter of $BC01 in index register Y.  Do I have this right?  My 6809ing
> is still quite a bit rusty.
> I'm still learning the great commands to include files into and extract
> files from RS-DOS disk image files, so I finally just created a text file
> with "123456789", changed the extension to "bin" and edited the resulting
> file named pclear0.bin with WINHEX changing the contents to the hex bytes
> shown in the data statement below.
> I was curious to see what the hex code machine language was doing also, so I
> thought I'd find out.  It didn't take me long to do since I had already
> compiled 6809dasm.  I used 6809dasm compiled and run in CYGWIN to get the
> resulting disassembly listing.
> Hope this helps.  Have fun!
> Kip
>
> -----Original Message-----
> From: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com] On
> Behalf Of Allen Huffman
> Subject: [Coco] Extended Color BASIC memory
>
> <portions deleted>
>
> 5
> FORA=0TO8:READA$:POKE1024+A,VAL("&H"+A$):NEXTA:EXEC1024:DATAC6,1,96,BC,1F,2,
> 7E,96,A3
>
> So... Someone wanna re-remind me what this actually does to accomplish
> PCLEAR 0?
>
> -
> Allen Huffman - PO Box 22031 - Clive IA 50325 - 515-999-0227 (vmail/TXT
> only) Sent from my iPad.
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco
>
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco
>