DivIDE programming model
------------------------
DivIDE interacts with ZX Spectrum using I/O accesses or automatically by
replacing original ZX-ROM with its own memory, when CPU fetch from an
entry-point is detected (and such behavior is somehow allowed).
DivIDE contains 8 kB ROM (could be not present, EPROM or EEPROM, in the third
case it's in-system reprogrammable) and 32-512 kB RAM. DivIDE can use system
from 3rd 8k bank of its RAM, this is suitable especially for safe system
development. Accidental EEPROM damage (due to unlucky out from crashed code)
is fused by the on-board jumper.
All ports are decoded using A0..A7 address-wires only. So you can access all
eight IDE-registers from so caled command block (rrr=0..7) at addresses
xxxx xxxx 101r rr11. Other divIDE features are controlled using dedicated
port at address xxxx xxxx 1110 0011, which belongs to divIDE CONTROL REGISTER.
IDE command block registers
---------------------------
[ DATA REGISTER (R/W) ]
xxxx xxxx 1010 0011, 0a3h, 163
This register serves for reading or writing sectors. It's 16-bit wide, so
divIDE is joining write-pairs into words and splitting readed words into bytes.
When reading from this register, each ODD access will return low byte of
readed word (while high byte is stored into latch and buffer pointer is shifted
to next word). Each EVEN access will return that high byte, stored in previous
read access to DATA REGISTER.
When writing to DATA REGISTER, each ODD access will store the byte into latch.
Each EVEN access will join the previously stored low portion with actual byte
(considered to be high portion), and this word is written into harddrive (and
next word in harddrive buffer is pointed).
After any access to non-data command block registers (rrr=1..7) or to divIDE
control register, following DATA REGISTER access is considered to be ODD.
Accesses outside divIDE ports cannot change the EVEN/ODD state. After reset or
power-on, this state is unknown.
[ ERROR REGISTER (R) / FEATURES REGISTER (W) ]
xxxx xxxx 1010 0111, 0a7h, 167
[ SECTOR COUNT REGISTER (R/W) ]
xxxx xxxx 1010 1011, 0abh, 171
[ SECTOR NUMBER REGISTER (R/W) (too LBA bits 0..7) ]
xxxx xxxx 1010 1111, 0afh, 175
[ CYLINDER LOW REGISTER (R/W) (too LBA bits 8..15) ]
xxxx xxxx 1011 0011, 0b3h, 179
[ CYLINDER HIGH REGISTER (R/W) (too LBA bits 16..23) ]
xxxx xxxx 1011 0111, 0b7h, 183
[ DRIVE/HEAD REGISTER (R/W) (too LBA bits 24..28) ]
xxxx xxxx 1011 1011, 0bbh, 187
[ STATUS REGISTER (R) / COMMAND REGISTER (W) ]
xxxx xxxx 1011 1111, 0bfh, 191
For detailed desription of mentioned registers have a look at
http://www.t13.org .
DivIDE control register
-----------------------
[ CONTROL REGISTER (Write Only) ]
xxxx xxxx 1110 0011, 0e3h, 227
This register is write-only (readed data will be unknown). All bits are reset to
'0' after each power-on. Unimplemented bits, marked 'X', should be zeroed for
future compatibility issues with more than 32kB RAM DivIDEs.
7 6 5 4 3 2 1 0
[ CONMEM , MAPRAM, X, X, X, X, BANK1, BANK0 ]
Bits BANK1 and BANK0 select the 8k bank, which normally appears in area
2000-3fffh, when divide memory is mapped.
Bit MAPRAM can be set to '1' only (when it's '1', only power-on can return it
to '0'). This bit allows 8k bank No.3 to act as EPROM/EEPROM, and write-protects
it. Set it when system image is loaded into bank No.3 and you want to safe probe
it till the next power-on. Set it also when you haven't EPROM/EEPROM, and you
want to use system. (Because u will possibly re-install it from MAPRAM mode, you
will need CONMEM to override writeprotect. Because after returning to BASIC you
rely on original ROM, you must reset automapper, which could be left in
undesired state by previous bus transactions. Because it could be done only by
fetching code from off-area, u must set DI, then call 1ffbh ('RET'), ten CONMEM, loading image,
releasing CONMEM and setting MAPRAM, then EI. Then the return is safe.)
Bit CONMEM forces EPROM/EEPROM to 0000-1fffh and by BANK 1..0 selected bank to
area 2000-3fffh, regardless to the actual divIDE state or to physical presence
of EPROM/EEPROM. Bank in area 2000-3fffh is always writable, and in 0000-1fffh
always appears EEPROM/EPROM, which is writable when EPROM jumper is open. Use
it in third-party utilities for loading system or modules, or in system's
auto-upgrade process.
Memory mapping
--------------
Memory mapping could be invoked manually (by setting CONMEM), or automatically
(CPU has fetched opcode form an entry-point). Automatic mapping is active
only if EPROM/EEPROM is present (jumper EPROM is closed) or bit MAPRAM is set.
Automatic mapping occurs at the begining of refresh cycle after fetching
opcodes (M1 cycle) from 0000h, 0008h, 0038h, 0066h, 04c6h and 0562h. It's
also mapped instantly (100ns after /MREQ of that fetch is falling down) after
executing opcode from area 3d00..3dffh. Memory is automatically disconnected in
refresh cycle of the instruction fetch from so called off-area, which is
1ff8-1fffh.
The one-instruction delay could be used to distinguish between nested calls to
the same place. For such trick you will place different instruction at the
entrypoint address, than is in original ZX ROM. So the first call wil execute
original instruction, but subsequent one will jump to another code, because
divIDE memory was already mapped, with its changed opcode. It's great for 100%
avoidance of nested NMI, which cannot be implemented using pure combinatorial
hardware workaround. It allows divIDE to use INT for timing, when divIDE code is
performed (external calls will map later divIDE off and continue in #38h
original service, but nested INTs can jump to another work, and mapping off is
of course undesirable there).
In automatic mapping, when MAPRAM is set, in location 0000h-1fffh appears
write-protected Bank No.3, instead of EEPROM/EPROM. The meaning-priority is
(from lowest to highest) EPROM jumper -> MAPRAM -> CONMEM. So because that
jumper is no more significant when MAPRAM or even CONMEM is set, it's used
to distinguish between fused or unfused EEPROM state for CONMEM mode.
So, when CONMEM is set, there is:
0000-1fffh - EEPROM/EPROM/NOTHING(if empty socket), and this area is
flash-writable if EPROM jumper is open.
2000-3fffh - 8k bank, selected by BANK 0..1 bits, always writable.
When MAPRAM is set, but CONMEM is zero, and entrypoint was reached:
0000-1fffh - Bank No.3, read-only
2000-3fffh - 8k bank, selected by BANK 0..1. If it's different from Bank No.3,
it's writable.
When MAPRAM is zero, CONMEM is zero, EPROM jumper is closed and entrypoint was
reached:
0000-1fffh - EEPROM/EPROM/NOTHING(if empty socket, so open jumper in this case),
read-only.
2000-3fffh - 8k bank, selected by BANK 0..1, always writable.
Otherwise, there's normal speccy memory layout. No modified ROM, no shit.
And, that's all, Folks! Do you think, it's too complex? Read it once again.
Do you still think, it's not sane? Feel free to ask me (Zilog), my email is
xcimbal@quick.cz. I hope you will enjoy this piece of hardware, i have spent
months for optimising this marvelous piece of shit.
With regards
Zilog
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