29 Nov
2007
29 Nov
'07
1:08 p.m.
On Nov 28, 2007 4:43 PM, Jules Richardson <julesrichardsonuk at yahoo.co.uk>
wrote:
I thought the speedup poke, at least in the CoCo involved reprogramming
the 6883 [1], and that the location of that was determined by the chips
themselves, not by the BASIC firmware.
[1] Ths 6883 SAM (Synchonous Address Multiplexer) chip is connected to
the processor address bus, but not the data bus. There are a number of
reserved addresses near the top of the memory space, which, by writing to
them, set and clear 1-bit registers in the SAM to select things like
graphics mode, video start address, memeory map type, and so on.
Hmm, interesting. Is that the one that was the same as the Dragon 32 in
the UK
(i.e. built from the 6809 app note)?
If so, I'll be bringing a Dragon 64 over at some point ( > 6 months), and
I
have the disk controller cartridge for it, along with the disk unit itself
and
quite a stack of software...
So yes, maybe I'm best off looking for a Coco (I assume 'Coco' == 'Color
Computer') right now, and then one day (assuming the lad's still keen)
'upgrading' him to a BBC micro, C64 etc...
As I understand it, the Color Computer / "CoCo" is very similar to the
Dragon 32/64, except with some differences in memory locations -- things
like the location of the "speedup poke" and so forth. I don't have the
slightest idea whether the CoCo version of OS-9 runs
on a Dragon.
I would not be at all suprised if it didn;t.
Although I'm in the UK and love the 6809, I';ve never done much serious
stuff with the Dragon. I have a few CoCos (including a CoCo3), though.
If you happen to try to get a US CoCo, be aware that there are a number of
levels to which they can be upgraded -- from 4K to 64K of memory and Color
BASIC vs. Extended Color BASIC. (Many 32K systems actually have 64K
I think you certainly want to find one with Extanded Color BASIC. It's
certainly needed for a disk system, and it's got all the high-res
graphics commands (the graphics _hardware_ is there on a Color BASIC
macbine, but you have to POKE about to get anything out of it).
Early machines (CoCo 1s?) had one 8K ROM for Color BASIC, and a second 8K
ROM that was added alongside it for Extended Color BASIC. Later machins
(CoCo 2s?) have one ROM socket that can be jumpered either for an 8K
Color BASIC ROM or a 16K Extended Color BASIC ROM.
installed, with half of it jumpered away.) The
keyboards are nice, though:
Some CoCo 1s had 32K RAM chips (== 64K chips with half defective). CoCo
2s normally have real 64K chips (actually 64K*4, 2 of them).
The power-on memory map is to have 32K RAM in the bottom half of the
address space, the top half of the spave being essentially ROM (16K for
the cartridge slot, then 8K Extended Color BASIC, then 8K COlour BASIC),
with I/O replacing part of that last ROM (this is all controlled by the
SAM chip, BTW). It's possible to program the SAM to disable the ROMs and
have 64K RAM (less the I/O space). Obviosuly this is only useful if you
have something in the RAM to execute!
you can take them completely apart for cleaning
without much effort.
The CoCo 3 is a very different beast inside. It's a great little computer
(wouldn't give mine up for the world), but something to stay away from if
understanding the design is a big deal: almost everything outside the
processor is implemented on a custom ASIC.
Indeed. This ASIC (called GIME (Grpahics, Interrupts, Memory, Enhancments)
iIRC) replaces the SAM and VDG chips, and does rather more. For example
you can have 512K memory, 640*192 graphics, 80 column text, true upper
and lower case, and so on. The CoCo 3 also has a 32K BASIC ROM.
There is some data on this ASIC in the CoCo 3 service manua, but nowhere
near as much as the data sheets for the SAM and VDG give for those chips.
-tony