Difference between revisions of "ASCII-EX mapper"

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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.
 
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.
  
−
== Principales ==
+
== 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 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).
 
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.
 
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.
  
 
{| class="wikitable"
 
{| class="wikitable"
 
|-
 
|-
−
! 15 || 14 || 13 || 12 || 11 || 10 || 9 || 8 || || 7 || 6 || 5 || 4 || 3 || 2 || 1 || 0
+
!colspan="8"| Higher byte
 +
|
 +
!colspan="8"| Lower byte
 +
|-
 +
! 15 || 14 || 13 || 12 || 11 || 10 || 9 || 8  
 +
|
 +
! 7 || 6 || 5 || 4 || 3 || 2 || 1 || 0
 
|-
 
|-
 
| 0 || 0 || 0 || 0
 
| 0 || 0 || 0 || 0
Line 21: Line 31:
  
 
Detection by emulators should be fairly easy (counting ROM write accesses).
 
Detection by emulators should be fairly easy (counting ROM write accesses).
 +
 +
== Format ==
  
 
Below, the parts in blue are the only additions to the standard ASCII8/16 format.
 
Below, the parts in blue are the only additions to the standard ASCII8/16 format.
  
−
== ASCII-EX16 mapper ==
+
=== ASCII-EX16 mapper ===
  
 
{| class="wikitable"
 
{| class="wikitable"
Line 35: Line 47:
  
 
* Maximum number of segments: 4096
 
* Maximum number of segments: 4096
−
* Maximum ROM size: 64 MB (1024 MB)
+
* Maximum ROM size: 64 MB
  
−
== ASCII-EX8 mapper ==
+
=== ASCII-EX8 mapper ===
  
 
{| class="wikitable"
 
{| class="wikitable"
Line 54: Line 66:
 
* Maximum ROM size: 32 MB
 
* Maximum ROM size: 32 MB
  
−
== Misc ==
+
== Appendix ==
−
 
 
−
=== Naming ===
 
−
If the ASCII-EX (extended ASCII) name poses any legal issue, we could simply called those mappers EX8 and EX16.
 
  
 
=== Bank switching cost ===
 
=== Bank switching cost ===
Line 63: Line 72:
  
 
  ; Direct access
 
  ; Direct access
 +
LD A,n      ;  8 t-states
 
  LD (nn),A  ; 14 t-states
 
  LD (nn),A  ; 14 t-states
 
   
 
   
 
  ; Indirect access
 
  ; Indirect access
 
  LD HL,nn    ; 11 t-states
 
  LD HL,nn    ; 11 t-states
−
  LD (HL),A   ; 8 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 doubled, but remains reasonable.
+
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.
−
 
 
−
=== 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 <tt>6000h</tt>, 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
 
  ; Direct access
 +
LD HL,nn    ; 11 t-states
 
  LD (nn),HL  ; 17 t-states
 
  LD (nn),HL  ; 17 t-states
 
   
 
   
 
  ; Indirect access
 
  ; Indirect access
 +
LD DE,nn    ; 11 t-states
 
  LD HL,nn    ; 11 t-states
 
  LD HL,nn    ; 11 t-states
 
  LD (HL),E  ;  8 t-states
 
  LD (HL),E  ;  8 t-states
 
  INC HL      ;  7 t-states
 
  INC HL      ;  7 t-states
 
  LD (HL),D  ;  8 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 ===
  
−
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.
+
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) /* ... */
 
  #define SET_BANK_SEGMENT(bank, segment) /* ... */
 
   
 
   
−
  // Make segment #12 visible through bank #1
+
  // Make segment #30 visible through bank #1
−
  SET_BANK_SEGMENT(1, 12);
+
  SET_BANK_SEGMENT(1, 30);
  
−
[[Category:Proposal]]
+
[[Category:Proposal]][[Category:Proposal/Mapper]]

Latest revision as of 00:07, 12 December 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.

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

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) 6000h (mirror: even address up to 67FEh) 0000h
1: 8000h~BFFFh (mirror: 0000h~3FFFh) 7000h (mirror: even address up to 77FEh) 0000h
  • Maximum number of segments: 4096
  • Maximum ROM size: 64 MB

ASCII-EX8 mapper

Bank (8kB) Switching address Initial segment
0: 4000h~5FFFh (mirror: C000h~DFFFh) 6000h (mirror: even address up to 67FEh) 0000h
1: 6000h~7FFFh (mirror: E000h~FFFFh) 6800h (mirror: even address up to 6FFEh) 0000h
2: 8000h~9FFFh (mirror: 0000h~1FFFh) 7000h (mirror: even address up to 77FEh) 0000h
3: A000h~BFFFh (mirror: 2000h~3FFFh) 7800h (mirror: even address up to 7FFEh) 0000h
  • 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);
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