Using 48 kB ROM unmapped with the LPE-FlashG 
Monday, December 15, 2008, 20:15

Introduction


Usually flash cartridges allow reading data only from Z80 pages 1 and 2 (address range $4000-$BFFF). With the LPE-FlashG it's also possible to run software that was written to run from Z80 page 0 (address range $0000-$3FFF). Because of this feature it's also possible to run 48 kB unmapped ROM software, like "Universe: Unknown" and "The Cure". In case of the LPE-FlashG a little bit of mapping is required. Why and how is described in this article...

The LPE-FlashG mapper


The mapper is ASCII8 and ASCII16 compatible, selectable with a jumper. General information about MegaROM mappers is described on:

http://bifi.msxnet.org/msxnet/tech/megaroms.html

The following table shows the Z80 paging and LPE-FlashG specifics on how the first 4 blocks are initialised:

Z80 Page 0 1 2
Mapper $0000-$1FFF $2000-$3FFF $4000-$5FFF $6000-$7FFF $8000-$9FFF $A000-$BFFF
ASCII8 ---- (0) ---- (0) $6000 (0) $6800 (1) $7000 (2) $7800 (3)
ASCII16 ---- (0) $6000 (0) $7000 (2)


The values between parenthesis are the initial block values. The hexadecimal values (indicated by the $-prefix) preceding them are the common switch addresses, if applicable.

Adapting the 48 kB unmapped ROM


To get 48 kB unmapped ROM software running from the LPE-FlashG we need to have the following configuration:

Z80 Page 0 1 2
Mapper $0000-$1FFF $2000-$3FFF $4000-$5FFF $6000-$7FFF $8000-$9FFF $A000-$BFFF
ASCII16 ---- (0) $6000 (1) $7000 (2)

As shown in the table, this 48 kB unmapped feature is only possible in ASCII16 mode. The value for page 1 should be 1 in stead of the initialised 0. To correct this, a little piece of additional code is required. The following source code (tniASM ready) shows how it's to be done:
		org	0
phase $4000
dw "AB",lpeInit,0,0
rst8: ds 3
lpeInit: ld a,1
ld ($6000),a
dephase

; over here, the rest of your code and data for page 0 is
; located

org $4000
dw "AB",init,0,0,0,0,0,0
init:

; over here, the rest of your code and data for pages 1 and
; 2 is located

Here's how it works. When the ROM is started under normal conditions, the system will look for the cartridge header on address $4000. The address on init is found and it'll jump to that address, causing it to start normally.

When the ROM is started from the LPE-FlashG block 0 is located in both pages 0 and 1. Block 2 is already correctly mapped in page 2. In this case the cartridge header is found and the address on lpeInit is found and executed. This special piece of code is fitted exactly before the init in block 1, where the real page 1 init code is located. The code at lpeInit switches page 1 to block 1, to restore the full 48 kB ROM. After that the address of init is reached AND the correct block is selected in page 1. This means the blocks are set up correctly with the first 48 kB ROM in unmapped style and the game will start as if it was started under normal conditions.

The 3 bytes at rst8 can be used for an optional RST $8 entry.

Final words


I hope this article will be useful when creating new 48 kB unmapped ROMs for MSX to run them from LPE-FlashG without the requirement to make changes later on.

Of course it's also possible to patch existing 48 kB unmapped ROMs, like "Universe: Unknown" and "The Cure" mentioned in the introduction, to run from LPE-FlashG. For these two titles, IPS patches are already available here and may be useful in creating your own patches for other 48 kB unmapped ROMs.
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