INES Mapper 018

iNES Mapper 018 represents the Jaleco SS8806 mapper used for Magic John (Japanese version of Totally Rad) and a couple other games.

This mapper appears to be connected only to A12-A14, A0-A1, and D0-D3, so the PRG bank and CHR bank numbers are split over two adjacent mapper numbers. Bits 0-3 are in the lower address of a pair (e.g. $xxx0 or $xxx2), and bits 4-7 are in the next higher address.

$8000-1: PRG $8000
3210 3210
|||| ||||
++++-++++- Select 8192 byte PRG ROM bank in $8000-$9FFF

$8002-3: PRG $A000
$9000-1: PRG $C000
(PRG $E000 is fixed to the last bank)

$A000-1: CHR $0000
3210 3210
|||| ||||
++++-++++- Select 1024 byte CHR ROM bank in $0000-$03FF

$A002-3: CHR $0400
$B000-1: CHR $0800
$B002-3: CHR $0C00
$C000-1: CHR $1000
$C002-3: CHR $1400
$D000-1: CHR $1800
$D002-3: CHR $1C00

The IRQ counter is a 16-bit down counter. Port $F001 controls how far the borrow propagates: If all counting bits wrap from $0 to $F, it doesn't modify the fixed bits before it but instead triggers an IRQ. Writes to $F000 or $F001 acknowledge the IRQ.

7654 3210  $F001: IRQ control
     ||||
     |||+- Enable (0: pause; 1: count)
     +++-- 0: Count down in bits 15-0 and hold no bits constant
           1: Count down in bits 11-0 and hold bits 15-12 constant
           2-3: Count down in bits 7-0 and hold bits 15-8 constant
           4-7: Count down in bits 3-0 and hold bits 15-4 constant

Alternate interpretation:

7654 3210  $F001: IRQ control
     ||||
     |||+- Enable (0: pause; 1: count)
     ||+-- Hold bits 15-12 constant
     |+--- Hold bits 15-8 constant
     +---- Hold bits 15-4 constant

This counter uses a reload value. All bits are reloaded, even the ones held constant.

• $E000: Set bits 3-0 of PRG
• $E001: Set bits 7-4 of counter
• $E002: Set bits 11-8 of counter
• $E003: Set bits 15-12 of counter

 Here are Disch's original notes:
 ========================
 =  Mapper 018          =
 ========================
 
 
 Example Games:
 --------------------------
 The Lord of King
 Magic John
 Pizza Pop
 
 
 Registers:
 ---------------------------
 
 Range,Mask:   $8000-FFFF, $F003
 
   $800x,$900x:  [.... PPPP]  PRG Regs
   $A00x-$D00x:  [.... CCCC]  CHR Regs
   $E00x:  [.... IIII]  IRQ Reload value
   $F000:  [.... ....]  IRQ Reset
   $F001:  [.... SSSE]  IRQ Control
      S = Size of IRQ counter
      E = Enable
 
   $F002:  [.... ..MM]  Mirroring
      %00 = Horz
      %01 = Vert
      %10 = 1ScA
      %11 = 1ScB
 
 
 CHR Setup:
 ---------------------------
 
 Only low 4 bits of written value significant [.... CCCC]
 
 2 regs combined to get an 8-bit page number
 
 $x000 or $x002 are the low 4 bits
 $x001 or $x003 are the high 4 bits
 
 
       $0000   $0400   $0800   $0C00   $1000   $1400   $1800   $1C00 
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |$A000+1|$A002+3|$B000+1|$B002+3|$C000+1|$C002+3|$D000+1|$D002+3|
     +-------+-------+-------+-------+-------+-------+-------+-------+
 
 
 PRG Setup:
 ---------------------------
 
 Same as CHR, $x000 low, $x001 high
 
       $8000   $A000   $C000   $E000 
     +-------+-------+-------+-------+
     |$8000+1|$8002+3|$9000+1| { -1} |
     +-------+-------+-------+-------+
 
 
 IRQ:
 ---------------------------
 
 16-bit IRQ Reload value is set via regs $E00x.  $E000 sets the low 4 bits, $E003 sets the high 4 bits.
 
 When enabled, the IRQ counter counts down every CPU cycle.  When it wraps, an IRQ is generated.
 
 The 'S' bits in the control reg determine the size of the IRQ counter.  It can be 4, 8, 12, or 16 bits wide:
 
   %000 = 16 bits wide
   %001 = 12 bits wide
   %01x = 8 bits wide
   %1xx = 4 bits wide
 
 If the counter is less than 16 bits, the high bits are not altered by IRQ counter clocking; they retain their
 value.
 
 Example:  if the IRQ counter contains $1232, and is in 4-bit mode, it counts like so:
 
   $1232
   $1231
   $1230
   $123F  <--- IRQ here
   $123E
    ...
 
 
 Any write to the reset reg ($F000) will copy the 16-bit reload value into the IRQ counter (full 16 bits are
 copied, regardless of current 'S' value).
 
 Any write to $F000 or $F001 will acknowledge the IRQ.