Looking at the memory map of the SQ80 offers a solution: Examine the
I/O space located between $6000 and $6fff - although a 4kB area is reserved
for I/O only 254 address locations are really used as the following list
shows:
-
$6000 to $60e2 (DOC)
-
$6400 to $640f (DUART)
-
$68f8, $68f0, $68e8, $68d8, $68b8, $6878 (DAC & Multiplexer)
-
$6c00 (Mapper)
-
$6e00 to $6e03 (FDC)
Since the DAC/Multiplexer decoding is tightly coupled to A7 to A0 it actually
occupies a whole page (256 bytes) which leads to the following calculation:
4096 bytes - (248 + 256) bytes = 3592 bytes
3592 bytes of memory space are just wasted due to improper address decoding!
Of course quite a tricky decoding is needed to implant additional ROM into
this area since the I/O areas have to be kept separated - but today, where
PALs and CPLDs are quite cheap that's no issue.
But we aren't limited to a single 4kB area - we easily can have 64kB
(or if needed up to 512kB) of additional ROM. How? Keep in mind that the
Mapper only uses Bit 0, the Bits 7 to 1 are free for use and can be connected
to the upper address lines of bigger EPROMs such as 27512 and above. Just
look at the following schematics:
As you can see, the bit pattern can be read back - and it has to be,
since only through that mechanism one can do independent manipulation of
each control bit without keeping the pattern stored somewhere in RAM (which
would cause interference with the original ROM routines). Now I hear you
cry, that this new Mapper would be incompatible to the original one - and
you are right. But luckily we have enough unused storage area in ROMHIGH
which can hold replacement routines which can be used instead of the original
"STA $6c00". The area I'm speaking of is $ffa4 to $ffdf offering 60 bytes
of free memory. Fortunately a "JSR address" occupies the same space
the STA command does so we easily can patch the ROM to access the following
routines:
mapseq PSHS A,CC ; rescue
A
LDA $6C00
; get bit pattern
ORA #$01
; map SEQRAM
BRA eomap
; saves 3 bytes :)
mapdos PSHS A,CC ; rescue A
LDA $6C00
; get bit pattern
ANDA #$FE
; map DOSRAM
eomap STA $6C00
PULS PC,CC,A ;
rescue registers
This takes 21 bytes, so we have another 39 bytes left. These we can use
for a bankswitching routine for accessing multiple 4kB blocks. Having a
single output multiplexer in mind I reserve bits 7 to 5 for audio channel
multiplexing and only use bits 4 to 1 for ROM block multiplexing:
page4k PSHS A,CC ; page number
to stack
LDA $6C00
; get bit pattern
ANDA #$E1
; keep mapper & audio mux bits
BRA eopage
; saves 1 byte :)
audiochn PSHS A,CC ; channel number to stack
LDA $6C00
; get bit pattern
ANDA #$1F
; keep mapper & ROM mux bits
eopage ORA ,S+ ; merge
in new mux pattern
PULS PC,CC
These routines use another 20 bytes so we still have 19 bytes left - try
to do that in PASCAL...
What can we do with this additional space? Well, I think of the following:
-
DOS routines giving the SQ80 the ability to read and write the world's
most famous file system
-
Single channel outputs
-
Master Keyboard abilities
-
New, bigger & better OS :)
Now, the ROM itself will have a funny layout since it can be only used
in the non-overlapping areas, so the following areas of each 4kB section
have to be left out:
-
$0000 to $00E2 (occupied by DOC)
-
$0400 to $040F (occupied by DUART)
-
$0800 to $08FF (occupied by DAC/MUX)
-
$0C00 (occupied by Mapper)
-
$0E00 to $0E03 (occupied by FDC)
If you are writing programs for this ROM area you have to take care that
your code doesn't range into these "forbidden" zones and you might have
to manoeuver around these by BRA.
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