Difference between revisions of "ASCII-EX mapper"
From MSX Game Library
(→ASCII-EX16 mapper) |
(→Misc) |
||
Line 85: | Line 85: | ||
// Make segment #12 visible through bank #1 | // Make segment #12 visible through bank #1 | ||
SET_BANK_SEGMENT(1, 12); | SET_BANK_SEGMENT(1, 12); | ||
+ | |||
+ | === Hardware === | ||
+ | |||
+ | User Pentaru's advice: | ||
+ | * AM29F032 32mbit flash 5V. | ||
+ | * MBM29F033C, which is probably cheaper. | ||
[[Category:Proposal]] | [[Category:Proposal]] |
Revision as of 22:56, 23 November 2023
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.
Contents
Principales
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.
A 16-bit segment register may allow to select one of the 65536 segments for each memory bank, increasing the maximum size to 512 MB (8 KB segment) or 1 GB (16 KB segment).
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 |
---|---|---|
0: 4000h~7FFFh (mirror: C000h~FFFFh) | Lower byte: 6000h (mirror: even address to 67FEh) Higher byte: 6001h (mirror: odd address to 67FFh) |
0000h |
1: 8000h~BFFFh (mirror: 0000h~3FFFh) | Lower byte: 7000h (mirror: even address to 77FEh) Higher byte: 7001h (mirror: odd address to 77FFh) |
0000h |
- Maximum number of segment: 65536
- Maximum size: 1 GB (1024 MB)
ASCII-EX8 mapper
Bank (8kB) | Switching address | Initial segment |
---|---|---|
0: 4000h~5FFFh (mirror: C000h~DFFFh) | Lower byte: 6000h (mirror: 6001h~67FFh) Higher byte: 8000h (mirror: 8001h~87FFh) |
0000h |
1: 6000h~7FFFh (mirror: E000h~FFFFh) | Lower byte: 6800h (mirror: 6801h~6FFFh) Higher byte: 8800h (mirror: 8801h~8FFFh) |
0000h |
2: 8000h~9FFFh (mirror: 0000h~1FFFh) | Lower byte: 7000h (mirror: 7001h~77FFh) Higher byte: 9000h (mirror: 9001h~97FFh) |
0000h |
3: A000h~BFFFh (mirror: 2000h~3FFFh) | Lower byte: 7800h (mirror: 7801h~7FFFh) Higher byte: 9800h (mirror: 9801h~9FFFh) |
0000h |
- Maximum number of segment: 65536
- Maximum size: 512 MB
Misc
Naming
If the ASCII-EX (extended ASCII) name poses any 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.
; Direct access LD (nn),A ; 14 t-states ; Indirect access LD HL,nn ; 11 t-states LD (HL),A ; 8 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 doubled, but remains reasonable.
Alternative
If the impact of a segment change in a bank is critical, an alternative would be to use the odd and even addresses of mapper to access the 2 bytes of the register. In this way, it would be sufficient, for example, to write a 16-bit value to address 6000h, to change the 2 bytes of the segment register of the first bank at once.
The minimum cost with direct access would then be 17 t-states instead of 28 with the proposed configuration. On the other hand, we risk losing compatibility with games that use odd addresses for segment changes (if any). The benefit-risk ratio is questionable.
; Direct access LD (nn),HL ; 17 t-states ; Indirect access LD HL,nn ; 11 t-states LD (HL),E ; 8 t-states INC HL ; 7 t-states LD (HL),D ; 8 t-states
MSXgl
MSXgl uses macros to encapsulate bank switching mechanisms. It would therefore be totally transparent for a user to switch from an ASCII8 or even Konami-SCC mapper, to an ASCII-EX8 mapper.
#define SET_BANK_SEGMENT(bank, segment) /* ... */ // Make segment #12 visible through bank #1 SET_BANK_SEGMENT(1, 12);
Hardware
User Pentaru's advice:
- AM29F032 32mbit flash 5V.
- MBM29F033C, which is probably cheaper.