Introduction
Yamanooto is a flash cartridge for MSX computers that includes an FPGA implementation of Konami's SCC chip, which provides wavetable sound together with "megarom" mapping for large rom games.
Yamanooto thus uses by default the K5 mapper, expanded to 8 bit to allow direct 2 Mbyte rom size. The register access is done in a compatible way, using the upper bytes of page 1 (4000h-7FFFh):
- 7FFFh (ENAR): to enable register interface
- 7FFEh (OFFR): offset register (more on this later)
- 7FFDh (CFGR); configuration bits
Note that upon reset, no register is readable (you get the rom contents). You need to write the ENAR register [bit 1] to 1 to enable the registers access.
Slots selection
In order to access the Yamanooto (and any cartridge) you need to use the MSX slot mechanism to switch page 1 to the logical slot (a number from 0 to 3). MSX cartridges fit physical slots where you plug them in. Some computers have one physical slot but most have more than one.
Usually first physical slot is logical slot 1 and second (when present) is logical slot 2, so probably a Yamanooto is accessed in logical slots 1 or 2.
Provided you know which logical slot the cartridge is, you use the ppi (Parallel Port Interface) of the MSX to switch page 1 to that slot. The PPI has a register at port A8h to switch pages to logical slots:
- bit 0-1: slot for page 0 (0000h-3FFFh)
- bit 2-3: slot for page 1 (4000h-7FFFh)
- bit 4-5: slot for page 2 (8000h-BFFFh)
- bit 6-7: slot for page 3 (C000h-FFFFh)
(bits in port A8h, AKA port 0 of the PPI)
So, i.e. to switch page 1 to slot 1 you need to keep bits 0-1 and 4-5 unchanged and replace bits 2-3 with value "01" (slot 1).
; switch page 1, desired slot value in B
IN A,(0A8h)
LD (orig_value),A ; save value to restore it later
AND 0F3h ; clear bits 2-3
LD C,A
LD A,B ; get slot value
RLCA
RLCA
OR C
DI ; disable ints before switch!
OUT (0A8h),A
<now desired slot is selected in page 1>
When you're done with the cartridge, you may need to restore the previous usage of page 1 before you reenable the interrupts:
; restore slots
LD A,(orig_value)
OUT (0A8h),A
EI
Setting the K5 mapper
The K5 mapper uses segments of 8 Kbyte length. Thus the 32 Kbyte cartridge area (pages 1-2, 4000h-BFFFh), are split in 4 banks. Each bank is "half a page"
To switch each bank to a given segment, you write the segment number in a "switch address" for each of the 4 banks:
- bank 0: 5000h-57FFh (usually 5000h)
- bank 1: 7000h-77FFh (usually 7000h)
- bank 2: 9000h-97FFh (usually 9000h)
- bank 3: B000h-B7FFh (usually B000h)
Note that the register area of the Yamanooto doesn't conflict with the switch addresses because the registers are in the upper area and the switch address area is in a lower part of the bank.
Once you write in the switch area, the bank is switched immediately, thus the following accesses are mapped to the new segment instead of the previously selected. If the executing code is in the affected bank, you may lose control because you're suddenly executing code in other segment.
The offset register (OFFR)
When K5 was first implemented by Konami in the original SCC chip, only 5 bits were implemented (hence the name) in each of the 4 banks, thus any of the 4 banks could be mapped to any of 32 segments.
As a segment is 8 kbyte long, this allowed for 32 * 8 kbyte = 256 kbyte rom.
Later, more bits were used, up to the full 8 bits possible in a single register. Yamanooto and other modern K5 implementations use all 8 bits which allow for 256 segments thus 256 * 8 kbyte = 2 Mbyte rom size.
In order to both overcome this limitation and provide a way of emulating a different rom for each of several games written in the the same rom space, an additional mechanism was added to allow for larger "flash cartridges".
The offset register changes which area that the "basic mapper" uses of the larger rom space. By adding an offset, we change which real address of the complete flash (which is bigger that 2 Mbyte) is accessed for "mapper segment 0" and all subsequent segments up to the mapper limit (2 Mbyte)
In the Yamanooto, the offset unit is 32kbyte, thus the mapper value is added four times the offset value:
segment address = mapper value + (4 * offset)
NOTE: the value is not applied until you actually write any of the corresponding mapper
As the normal mapper functionality is preserved, the offset register may be used to run a game with resides in a part (not only the beginning) of the large flash rom of the Yamanooto. The procedure would be:
- Switch page 1 to the correct slot (if needed)
- Enable the register access and set the OFFR to the game offset (in 32 kbyte units)
- Reset the mapper to initial values (usually 0-1-2-3). This applies the offset to all banks so you'll probably will want to do this with code in ram
- Do a fake reboot
A possible code for steps 1-2 would look like that this:
LD B,1 ; for slot 1
CALL set_page1 ; if you're running with page 1 in ram
LD HL,7FFFh
LD (HL),1
DEC HL
LD (HL),<offset value>
DEC HL
LD (HL),0 ; clear ROMDIS (see note)
Following you should reset the mapper and perform a fake reset (CALL 0) or simulate the cartridge boot in any manner.
Note that if you're running in some code outside of the Yamanooto flash (i.e. a dos command) and you've skipped the flash boot with the DEL key, you need to reenable the rom boot clearing the bit ROMDIS in CFGR (7FFDh). See the manual
To reset the mapper after changing the offset, you need to switch all four banks:
- Ensure pages 1-2 are mapped to correct slot
- Write 0 to switch address of bank 0 (5000h <= 0)
- Write 1 to switch address of bank 1 (7000h <= 1)
- Write 2 to switch address of bank 2 (9000h <= 2)
- Write 3 to switch address of bank 3 (B000h <= 3)
Further considerations
The brave would probably have already noticed that the offset register can be used to create large games that use the full 8 Mbyte size of the flash. You can even have each of the banks in a different offset. Any bank can be ANYWHERE because each bank is applied a new offset ONLY when switched!
The occurrence of a hardware reset is safe and will return control to the offset 0 (the normal start of the game or menu) because the mapper reset is done with offset 0 (i.e. actual 0-1-2-3)
In case of a menu selector, code can choose to restore the previously selected game or not by reading the OFFR register (which survives reset)
Have a nice time programming and make great games or compilations with Yamanooto!