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| [[iNES Mapper 073]] is used to represent the Konami [[VRC3]] mapper. | | [[iNES Mapper 073]] is used to represent the Konami [[VRC3]] mapper. |
| Here are Disch's original notes:
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| ========================
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| = Mapper 073 =
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| ========================
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|
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| aka
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| --------------------------
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| VRC3
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|
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|
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| Example Games:
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| --------------------------
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| Salamander
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|
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|
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| Registers:
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| --------------------------
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|
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| Range,Mask: $8000-FFFF, $F000
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|
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| $8000: [.... IIII] Bits 0- 3 of IRQ reload value
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| $9000: [.... IIII] Bits 4- 7 of IRQ reload value
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| $A000: [.... IIII] Bits 8-11 of IRQ reload value
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| $B000: [.... IIII] Bits 12-15 of IRQ reload value
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|
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| $C000: [.... .MEA] IRQ Control
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| M = IRQ Mode (0=16-bit mode, 1=8-bit mode)
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| E = IRQ Enable (0=disabled, 1=enabled)
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| A = Enable-on-Acknowledge (see IRQ section)
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|
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| $D000: [.... ....] IRQ Acknowledge (see IRQ section)
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|
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| $F000: [.... PPPP] PRG Select (16k @ $8000)
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|
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|
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| PRG Setup:
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| ---------------------------
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|
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| $8000 $A000 $C000 $E000
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| +---------------+---------------+
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| | $F000 | { -1} |
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| +---------------+---------------+
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|
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|
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| IRQs:
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| ---------------------------
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|
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| VRC3 IRQs operate differently from other VRCs. The counter is 16 bits instead of 8 bits, and there is no
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| scanline mode -- only CPU cycle mode. Other aspects, however, are very similar.
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|
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| $8000-B000 set the 16-bit reload value (not the actual IRQ counter). When $C000 is written to with the 'E'
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| bit set, the reload value is copied into the actual IRQ counter.
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|
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| When enabled, the IRQ counter will increment by 1 every CPU cycle until it wraps, at which point the IRQ
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| counter is reloaded with the reload value (relevent bits only! see Modes below) and an IRQ is tripped.
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|
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| Any write to $C000 or $D000 will acknowledge the IRQ.
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|
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| Any write to $D000 will also copy the 'A' control bit to the 'E' control bit... enabling or disabling IRQs.
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| This does not change the contents of the IRQ counter.
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|
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|
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| Modes:
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| ---------------------------
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| There are 8-bit and 16-bit modes for the IRQ counter, as controlled by the 'M' bit in $C000.
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|
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| In 16-bit mode (M=0):
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| - Counter is a full 16-bits.
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| - IRQ is triggered when IRQ counter is incremented from $FFFF
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|
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|
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| In 8-bit mode (M=1):
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| - Only the low 8-bit bits of counter are used
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| - IRQ is triggered when low 8 bits of IRQ counter are incremented from $FF
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| - Incrementing the low bits *never* alters the high bits of the counter
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| - When low 8 bits wrap, only the low 8 bits are copied from the reload value... high bits remain unchanged
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| - Reloading via $C000 write will still reload all 16 bits.
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