ASCII-EX mapper

Here's a proposal for a mapper format operating with a 16-bit segment register and allowing ROMs of up to 512 MB (8 KB segment) or 1 GB (16 KB segment).

A 16-bit segment register may allow to select one of the 65536 segments for each memory bank.

The idea is to use the ASCII8/16 mapper format for the low byte of the segment register and to use the 8000h~9FFFh memory space unused by ASCII to manage the high byte. This way, we could build hardware capable of running standard ASCII8/16 ROMs in addition to the new 16-bit format.

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

Below, the parts in blue are the only additions to the standard ASCII8/16 format.

ASCII-EX16 mapper

Bank (16kB)	Switching address	Initial segment
4000h~7FFFh (mirror: C000h~FFFFh)	Lower byte: 6000h (mirrors: 6001h~67FFh) Higher byte: 8000h (mirror: 8001h~87FFh)	0000h
8000h~BFFFh (mirror: 0000h~3FFFh)	Lower byte: 7000h (mirrors: 7001h~77FFh) Higher byte: 9000h (mirror: 9001h~97FFh)	0000h

• Maximum number of segment: 65536
• Maximum size: 1 GB (1024 MB)

ASCII-EX8 mapper

Bank (8kB)	Switching address	Initial segment
4000h~5FFFh (mirror: C000h~DFFFh)	Lower byte: 6000h (mirrors: 6001h~67FFh) Higher byte: 8000h (mirror: 8001h~87FFh)	0000h
6000h~7FFFh (mirror: E000h~FFFFh)	Lower byte: 6800h (mirrors: 6801h~6FFFh) Higher byte: 8800h (mirror: 8801h~8FFFh)	0000h
8000h~9FFFh (mirror: 0000h~1FFFh)	Lower byte: 7000h (mirrors: 7001h~77FFh) Higher byte: 9000h (mirror: 9001h~97FFh)	0000h
A000h~BFFFh (mirror: 2000h~3FFFh)	Lower byte: 7800h (mirrors: 7801h~7FFFh) Higher byte: 9800h (mirror: 9801h~9FFFh)	0000h

Misc

Naming

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

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.

LD (nn), A ; 14 t-states
; or
LD HL,nn
LD (HL),A ; 19 t-states (11 + 8)