INES Mapper 021

iNES Mapper 021 is used to represent the VRC4a and VRC4c variants of the Konami VRC4 mapper.

 Here are Disch's original notes:
 ========================
 =  Mapper 021          =
 =       + 023          =
 =       + 025          =  
 ========================
 
 aka
 --------------------------
 VRC4
 
 
 
 Example Games:
 --------------------------
 Wai Wai World 2          (021)
 Ganbare Goemon Gaiden 2  (021)
 Boku Dracula-kun         (023)
 Tiny Toon Adventures (J) (023)
 Gradius 2 (J)            (025)
 Bio Miracle Bokutte Upa  (025)
 
 
 
 Multiple numbers, just one mapper:
 --------------------------
 These three mapper numbers (021, 023, 025) collectively represent different wiring variations of the same mapper:
 VRC4.  Each variation operates exactly the same, only the registers used are different because they all use
 different address lines.  Some lines are even reversed from the norm.
 
    variant   lines     registers                       Mapper Number
    =================================================================
    VRC4a:    A1, A2    $x000, $x002, $x004, $x006      021
    VRC4b:    A1, A0    $x000, $x002, $x001, $x003      025
    VRC4c:    A6, A7    $x000, $x040, $x080, $x0C0      021
    VRC4d:    A3, A2    $x000, $x008, $x004, $x00C      025
    VRC4e:    A2, A3    $x000, $x004, $x008, $x00C      023
    VRC4f:    A0, A1    $x000, $x001, $x002, $x003      023  * see below *
 
 
 This doc will use the 'VRC4a' registers (0,2,4,6) in all following register descriptions.  For other
 variants, use the above chart to convert.
 
 I'm unsure whether or not 'VRC4f' really exists.  It seems to use the same registers are mapper 023's VRC2
 counterpart (see 022.txt) but also has IRQ functionality (appears to be used by Tiny Toon Adventures).  Could
 it be that 023 is really VRC4 and not a VRC4+VRC2 mix?
 
 
 Registers:
 --------------------------
 Some registers are mirrored across several addresses.  For example, writing to $9004 has the same effect as
 writing to $9006.
 
 $8000-$8006:  [...P PPPP]   PRG Reg 0
 $9000,$9002:  [.... ..MM]   Mirroring:
    %00 = Vert
    %01 = Horz
    %10 = 1ScA
    %11 = 1ScB
 
 $9004,$9006:  [.... ..M.]   PRG Swap Mode Select
 $A000-$A006:  [...P PPPP]   PRG Reg 1
 $B000-$E006:  [.... CCCC]   CHR Regs (see CHR Setup)
 
 $F000+$F002:  [.... IIII]   IRQ Reload Value (see IRQ section)
 $F004         [.... .MEA]   IRQ Control (see IRQ section)
 $F006         [.... ....]   IRQ Acknowledge (see IRQ section)
 
 
 PRG Setup:
 --------------------------
 There are two PRG modes, which can be seleted via $9004.
 
                $8000   $A000   $C000   $E000  
              +-------+-------+-------+-------+
 PRG Mode 0:  | $8000 | $A000 | { -2} | { -1} |
              +-------+-------+-------+-------+
 PRG Mode 1:  | { -2} | $A000 | $8000 | { -1} |
              +-------+-------+-------+-------+
 
 
 CHR Setup:
 --------------------------
 The VRC4 only has 4 data pins for CHR Regs.  To compensate, two CHR regs are combined to form a single page
 number.  One reg contains the high 4 bits and the other reg contains the low 4 bits (allowing for 8-bit page
 numbers)
 
 Example:  $B000+$B002  select 1k CHR page @ $0000
     if   $B000=$03
     and  $B002=$01
     then use page $13
 
 
       $0000   $0400   $0800   $0C00   $1000   $1400   $1800   $1C00 
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |$B000+2|$B004+6|$C000+2|$C004+6|$D000+2|$D004+6|$E000+2|$E004+6|
     +-------+-------+-------+-------+-------+-------+-------+-------+
 
 
 --------------------------------------------------
 --------------------------------------------------
 
 
 VRC IRQs:
 --------------------------
 VRC IRQ logic is shared by VRC4, VRC6, and VRC7.  IRQs for all of those mappers operate exactly the same way.
 Therefore, this section applies to all of those mappers (other docs refer here).
 
 One thing in paticular to note with VRC4 that is different from VRC6, VRC7 is that the reload register is
 split in two just as CHR regs are.  $F000 specifies the low 4 bits of the reload value, and $F002 specfies
 the high 4 bits.  This only happens in VRC4.   VRC6 and VRC7 have a single 8-bit register to specify the
 reload value.  The rest of this doc will refer to this reload value as a single register.
 
 
 Notes:
 --------------------------
 VRC IRQs are unique in that they simulate a scanline counter, without actually counting scanlines.  The IRQ
 counter is actually a CPU cycle counter, with a prescaler that divides clocks by ~113.666667 CPU cycles (one
 NTSC scanline).  This results in the IRQ counter being clocked once per scanline -- however unlike true
 scanline counters, it will be clocked even when the PPU is inactive, and even during VBlank!
 
 
 
 Registers:
 --------------------------
 
 There are 3 registers relevant to VRC IRQs.  See respective mapper doc for which register corresponds to
 which address:
 
   IRQ Reload:       [IIII IIII]
      This register specifies the counter reload value.  It does not affect the counter itself.
 
   IRQ Control:      [.... .MEA]
      M = IRQ Mode (0=scanline mode, 1=CPU cycle mode)
      E = Enable (0=disabled, 1=enabled)
      A = Enable-on-acknowledge (see below)
 
      - If 'E' is written as set, the IRQ counter will be immediately reloaded with the reload value, and the
 prescaler will be reset.  IRQs will also be enabled.
      - If 'E' is written as clear, the IRQ counter and prescaler are not changed, and IRQs are disabled
      - Any write to this register will acknowledge the IRQ.
 
 
   IRQ Acknowledge:  [.... ....]
      Any write to this register will acknowledge the IRQ.  In addition, the 'A' control bit is copied to the
 'E' control bit (enabling or disabling IRQs).  No write to this register will change the state of the IRQ
 counter or prescaler.
 
 
 Operation:
 --------------------------
 
 When in scanline mode ('M' control bit clear), a prescaler divides the passing CPU cycles by 114, 114, then
 113 (and then repeats that pattern).  This averages 113 + 2/3 CPU cycles (1 NTSC scanline).  When the
 prescaler is reset, the sequence is reset, and it will be 114 CPU cycles until the next IRQ counter clock.
 
 A simple way to emulate prescaler behavior is to have it reset to 341, and subtract 3 every CPU cycle.  When
 it drops to or below 0, increment it by 341 and clock the IRQ counter once.  This will produce the
 114,114,113 repeating pattern.
 
 When in cycle mode ('M' control bit set), the prescaler is effectively bypassed, and the IRQ counter gets
 clocked every CPU cycle.  In this mode, the prescaler remains unchanged by passing CPU cycles.
 
 If IRQs are disabled, neither the prescaler nor IRQ counter get clocked.
 
 
 When the IRQ counter is clocked:
  - If IRQ counter = $FF...
     a) reload IRQ counter with reload value
     b) trip IRQ
 
  - otherwise...
     a) increment IRQ counter by 1