INES Mapper 064

 Here are Disch's original notes:  
 ========================
 =  Mapper 064          =
 ========================
 
 
 aka
 --------------------------
 Tengen RAMBO-1
 
 
 
 Example Games:
 --------------------------
 Klax
 Skull and Crossbones
 Shinobi
 
 
 Notes:
 --------------------------
 This mapper is very similar to MMC3.  It uses a similar swapping system, but adds a little functionality.
 
 IRQs are set up similar as well... but have some major differences.
 
 This is one of those mappers that is a big pain to implement in an emu -- especially since so few games use
 it.  And the games that use it really blow hard.
 
 
 Registers:
 --------------------------
 
 Range,Mask:   $8000-FFFF, $E001
 
   $8000:  [CPK. AAAA]
     C = CHR mode select
     P = PRG mode select
     K = full 1k CHR mode select (see CHR setup)
     A = Address for use with $8001
 
   $8001:  [DDDD DDDD]   Data port
     R:0 ->  CHR reg 0
     R:1 ->  CHR reg 1
     R:2 ->  CHR reg 2
     R:3 ->  CHR reg 3
     R:4 ->  CHR reg 4
     R:5 ->  CHR reg 5
     R:6 ->  PRG reg 0
     R:7 ->  PRG reg 1
     R:8 ->  CHR reg 6
     R:9 ->  CHR reg 7
        R:A - R:E not used
     R:F ->  PRG reg 2
 
   $A000:  [.... ...M]   Mirroring
     0 = Vert
     1 = Horz
 
 
 
   $C000:  [IIII IIII]   IRQ Reload value
   $C001:  [.... ...M]   IRQ Mode select and reset
     0 = Scanline (A12) mode
     1 = Cycle mode
 
   $E000:  [.... ....]   IRQ Acknowledge/Disable
   $E001:  [.... ....]   IRQ Enable
 
 
 PRG Setup:
 ---------------------------
 
 PRG mode is selected via $8000.6
 
                $8000   $A000   $C000   $E000  
              +-------+-------+-------+-------+
 PRG Mode 0:  |  R:6  |  R:7  |  R:F  | { -1} |
              +-------+-------+-------+-------+
 PRG Mode 1:  |  R:F  |  R:6  |  R:7  | { -1} |
              +-------+-------+-------+-------+
 
 
 CHR Setup:
 ---------------------------
 
 $8000 has 2 bits to configure CHR modes.  Therefore there are effectively 4 CHR modes.
 
   $8000:  [CPK. AAAA]   <---  C and K bits relevant to CHR
 
 
               $0000   $0400   $0800   $0C00   $1000   $1400   $1800   $1C00 
             +---------------+---------------+-------+-------+-------+-------+
 C=0, K=0    |     <R:0>     |     <R:1>     |  R:2  |  R:3  |  R:4  |  R:5  |
             +---------------+---------------+-------+-------+-------+-------+
 C=0, K=1    |  R:0  |  R:8  |  R:1  |  R:9  |  R:2  |  R:3  |  R:4  |  R:5  |
             +-------+-------+-------+-------+---------------+---------------+
 C=1, K=0    |  R:2  |  R:3  |  R:4  |  R:5  |     <R:0>     |     <R:1>     |
             +-------+-------+-------+-------+---------------+---------------+
 C=1, K=1    |  R:2  |  R:3  |  R:4  |  R:5  |  R:0  |  R:8  |  R:1  |  R:9  |
             +-------+-------+-------+-------+-------+-------+-------+-------+
 
 
 
 IRQs:
 ---------------------------
 
 There are two separate IRQ modes.  One uses A12 to count scanlines in a manner just like MMC3 does (see
 mapper 004 for details on how scanlines are counted and the restrictions involved).  The other mode uses CPU
 cycles with a 4-step prescalar (so the IRQ counter gets clocked every 4 CPU cycles).
 
 Regardless of the mode used to clock the counter... every time the counter is clocked, the following actions
 occur:
 
 - If Reset reg ($C001) was written to after previous clock...
      a)  reload IRQ counter with IRQ Reload value **PLUS ONE**
 
 - Otherwise... If IRQ Counter is 0...
      a)  reload IRQ counter with IRQ Reload value
 
 - Otherwise...
      a)  Decrement IRQ counter by 1
      b)  If IRQ counter is now 0 and IRQs are enabled, trigger IRQ
 
 
 Just like with MMC3, the counter is clocked and updated even when IRQs are disabled -- however IRQs will only
 be triggered when enabled.
 
 Note about the plus one:  I'm not sure if 1 is really added or if there's simply an additional 1 clock delay
 before the IRQ counter is updated.  From a software standpoint, it doesn't really matter -- adding the
 additional 1 works without any side-effects.
 
 
 Registers involved with IRQs:
 ---------------------------
 
   $C000:   [IIII IIII] - IRQ Reload value
 
   $C001:   [.... ...M] - IRQ Reset reg, mode select
     0 = Scanline mode (A12)
     1 = CPU Cycle mode (with prescalar)
 
     Any write to this register will make it so that the IRQ counter will reload with the reload value +1 on
 its next clock.  Whether or not writing to this register clears the IRQ counter like it does with MMC3 isn't
 known... and doesn't matter, since it's reloaded later anyway.
 
     Also, any write to this register will reset the CPU cycle prescalar (so that it will be 4 CPU cycles
 until the next clock).
 
   $E000:   [.... ....] - IRQ Acknowledge/Disable
     Any write to this register will acknowledge the pending IRQ, and disable IRQs
 
   $E001:   [.... ....] - IRQ Enable
     Any write to this register will enable IRQs
 
 
 
 A note about IRQs:
 ------------------
 
   Scanline IRQs seem to trip a little later than they do on the MMC3.  It looks like about a 5 dot delay
 from the normal MMC3 IRQ time (265 instead of 260).  Failure to put in this delay results in shaking and
 other graphical quirks in some games... notably Klax.  This delay also seems to exist for CPU cycle driven
 IRQs (Skull & Crossbones will suffer without it).  Perhaps the RAMBO-1's IRQ generating hardware is a little
 slower than usual?
 
   Apart from that timing difference, A12 clocks RAMBO-1's IRQ counter just exactly like it does MMC3, so all
 the notes about A12, $2006/7, etc from the mapper 004 documentation apply to this mapper as well.