NEO mapper v1

Here's a proposal for a mapper format operating with a 16-bit segment register and allowing ROMs larger than the 2/4 MB limit of classic mappers.

Principles

The idea is to extend the ASCII mapper format using a 16-bit segment register so we could build hardware capable of running standard ASCII8/16 ROMs in addition to the new 16-bit format.

The lower byte of 16-bit register is mapped to even address of bank switch memory space while higher byte is mapped to odd ones. This means that older programs, which use the default even addresses (like 6000h), will be able to use this new mapper as if it had an 8-bit register (the low byte of the 16-bit register).

For the time being, we propose to reserve the 4 most significant bits for future extensions for the format (such as SRAM or sound chips support for example). This leaves 12 bits to select which segment is visible in each bank, for a maximum of 4096 segments. The reserved bits, must be set to 0.

Higher byte									Lower byte
15	14	13	12	11	10	9	8		7	6	5	4	3	2	1	0
0	0	0	0	Segment MSB					Segment LSB

The maximum ROM size is therefore 32 MB (for 8 KB segments) or 64 MB (for 16 KB segments).

Detection by emulators should be fairly easy (counting ROM write accesses).

Format

NEO-16 mapper

Bank (16kB)	Switching address	Initial segment
0: 0000h~3FFFh	9000h	0003h
1: 4000h~7FFFh	6000h	0000h
2: 8000h~BFFFh	7000h	0001h
3: C000h~FFFFh	8000h	0002h

• Maximum number of segments: 4096
• Maximum ROM size: 64 MB

NEO-8 mapper

Bank (8kB)	Switching address	Initial segment
0: 0000h~1FFFh	9000h	0006h
1: 2000h~3FFFh	9800h	0007h
2: 4000h~5FFFh	6000h	0000h
3: 6000h~7FFFh	6800h	0001h
4: 8000h~9FFFh	7000h	0002h
5: A000h~BFFFh	7800h	0003h
6: C000h~DFFFh	8000h	0004h
7: E000h~FFFFh	8800h	0005h

• Maximum number of segments: 4096
• Maximum ROM size: 32 MB

Appendix

Bank switching cost

The cost of switching only the lower or higher byte of the 16-bit segment register is the same that switching segment for standard ASCII mappers.

; Direct access
LD A,n      ;  8 t-states
LD (nn),A   ; 14 t-states

; Indirect access
LD HL,nn    ; 11 t-states
LD (HL),n   ; 11 t-states

Although a program can avoid having to change the 2 bytes of the segment register at once, there are cases where this may be necessary. In such cases, the cost is higher, but remains reasonable.

; Direct access
LD HL,nn    ; 11 t-states
LD (nn),HL  ; 17 t-states

; Indirect access
LD DE,nn    ; 11 t-states
LD HL,nn    ; 11 t-states
LD (HL),E   ;  8 t-states
INC HL      ;  7 t-states
LD (HL),D   ;  8 t-states

Naming

If the ASCII-EX (extended ASCII) name poses any legal issue, we could simply called those mappers EX8 and EX16.

MSXgl

MSXgl uses macros to encapsulate bank switching mechanisms. It would therefore be totally transparent for a user to switch, for example, from an ASCII8 or even Konami-SCC mapper, to an ASCII-EX8 mapper.

#define SET_BANK_SEGMENT(bank, segment) /* ... */

// Make segment #30 visible through bank #1
SET_BANK_SEGMENT(1, 30);