RAMBO-1

For the mapper used in the game "Rambo", see UxROM.

The Tengen RAMBO-1 is an ASIC mapper, canonically designated as mapper 64. This mapper is basically Tengen's version of the MMC3, but with some extra features. The RAMBO-1 came as a 40-pin PDIP. A variant with different mirroring control is mapper 158.

Overview

• PRG ROM size: Up to 256 KiB
• PRG ROM bank size: 8 KiB
• PRG RAM: None
• CHR capacity: Up to 256 KiB ROM
• CHR bank size: All 1 KiB, or two 2 KiB and four 1 KiB
• Nametable mirroring: Controlled by mapper
• Subject to bus conflicts: No

Banks

• CPU $8000-$9FFF: 8 KiB switchable PRG ROM bank
• CPU $A000-$BFFF: 8 KiB switchable PRG ROM bank
• CPU $C000-$DFFF: 8 KiB switchable PRG ROM bank
• CPU $E000-$FFFF: 8 KiB PRG ROM bank, fixed to the last bank
• PPU -- Three selectable configurations:
  1. 1 KiB switchable CHR banks at $0000, $0400, $0800, $0C00, $1000, $1400, $1800, $1C00
  2. 2 KiB switchable CHR banks at $0000, $0800; 1 KiB switchable CHR banks at $1000, $1400, $1800, $1C00
  3. 2 KiB switchable CHR banks at $1000, $1800; 1 KiB switchable CHR banks at $0000, $0400, $0800, $0C00

Registers

The RAMBO-1 has four pairs of registers at $8000-$9FFF, $A000-$BFFF, $C000-$DFFF, and $E000-$FFFF - even addresses ($8000, $8002, etc.) select the low register and odd addresses ($8001, $8003, etc.) select the high register in each pair.

Bank select ($8000-$9FFE, even)

7  bit  0
---- ----
CPKx RRRR
|||  ||||
|||  ++++- Specify which bank register to update on next write to Bank Data register
|||        0: Select 2 (K=0) or 1 (K=1) KiB CHR bank at PPU $0000 (or $1000);
|||        1: Select 2 (K=0) or 1 (K=1) KiB CHR bank at PPU $0800 (or $1800);
|||        2: Select 1 KiB CHR bank at PPU $1000-$13FF (or $0000-$03FF);
|||        3: Select 1 KiB CHR bank at PPU $1400-$17FF (or $0400-$07FF);
|||        4: Select 1 KiB CHR bank at PPU $1800-$1BFF (or $0800-$0BFF);
|||        5: Select 1 KiB CHR bank at PPU $1C00-$1FFF (or $0C00-$0FFF);
|||        6: Select 8 KiB PRG ROM bank at $8000-$9FFF (or $A000-$BFFF);
|||        7: Select 8 KiB PRG ROM bank at $A000-$BFFF (or $C000-$DFFF);
|||        8: If K=1, Select 1 KiB CHR bank at PPU $0400 (or $1400);
|||        9: If K=1, Select 1 KiB CHR bank at PPU $0C00 (or $1C00);
|||        F: Select 8 KiB PRG ROM bank at $C000-$DFFF (or $8000-$9FFF);
||+------- Full 1 KiB CHR bank mode (0: two 2 KiB banks at $0000-$0FFF (or $1000-$1FFF),
||                                 1: four 1 KiB banks at $0000-$0FFF (or $1000-$1FFF))
|+-------- PRG ROM bank mode (0: $8000-$9FFF uses bank selected with R:6,
|                                $A000-$BFFF uses bank selected with R:7,
|                                $C000-$DFFF uses bank selected with R:F;
|                             1: $8000-$9FFF uses bank selected with R:F,
|                                $A000-$BFFF uses bank selected with R:6, -- NOT 7
|                                $C000-$DFFF uses bank selected with R:7) -- NOT 6
+--------- CHR A12 inversion (0: two 2 KiB banks (or four 1 KiB banks) at $0000-$0FFF,
                                 four 1 KiB banks at $1000-$1FFF;
                              1: two 2 KiB banks (or four 1 KiB banks) at $1000-$1FFF,
                                 four 1 KiB banks at $0000-$0FFF)

In PRG ROM bank mode 1, the functions of registers 6 and 7 are backward compared to the corresponding mode of MMC3.

Bank data ($8001-$9FFF, odd)

7  bit  0
---- ----
DDDD DDDD
|||| ||||
++++-++++- New bank value, based on last value written to Bank select register (mentioned above)

Mirroring ($A000-$BFFE, even)

7  bit  0
---- ----
xxxx xxxM
        |
        +- Mirroring (0: vertical; 1: horizontal)

This applies to mapper 64 only (see Variants below).

IRQ latch ($C000-$DFFE, even)

7  bit  0
---- ----
DDDD DDDD
|||| ||||
++++-++++- IRQ latch value

This register specifies the IRQ counter reload value. When the IRQ counter is zero (or a reload is requested through $C001), this value will be copied into the IRQ counter at the end of the current scanline.

IRQ mode select / reload ($C001-$DFFF, odd)

7  bit  0
---- ----
xxxx xxxM
        |
        +- IRQ mode select (0: Scanline Mode, 1: CPU Cycle Mode)

Writing to this register also clears the IRQ counter so that it will be reloaded at next clock, or the next scanline, depending on the selected mode. This also resets the prescaler in cycle mode, so the next clock will occur 4 cycles later.

IRQ acknowledge / disable ($E000-$FFFE, even)

7  bit  0
---- ----
xxxx xxxx

Writing any value to this register will disable counter interrupts AND acknowledge any pending interrupts.

IRQ enable ($E001-$FFFF, odd)

7  bit  0
---- ----
xxxx xxxx

Writing any value to this register will enable counter interrupts.

IRQ counter operation

The counter can be set to either scanline mode or cpu cycle mode.

In scanline mode, the counter is clocked using a very similar method to that used by the MMC3, and follows the same restrictions. In comparison to the MMC3, the actual interrupt triggers slightly later. Specifically, it is delayed until M2 falls twice after the PPU A12 rise that would have triggered the MMC3 interrupt.

In CPU cycle mode, the counter is clocked every 4 CPU cycles. The actual interrupt triggers one M2 cycle later than one would naively expect.

Whichever method is being used, the counter behaves the following way:

• IF $C001 was written to after previous clock
  • reload IRQ counter with IRQ Reload value PLUS ONE (see note)
• ELSE IF IRQ counter is 0
  • reload IRQ counter with IRQ Reload value
• ELSE
  • Decrement IRQ counter by 1
  • IF IRQ counter is now 0 AND IRQs are enabled
    • wait one M2 cycle, then trigger IRQ

Note: When $C001 is written to, it's unknown as to whether reload+1 is written to the counter, or if the counter just takes an extra clock to start counting.

Variants

Mapper 158, used for Alien Syndrome, has mirroring like mapper 118 (TLSROM), where CIRAM A10 is connected to CHR A17, and bit 7 of each CHR bank mapped into PPU $0000-$0FFF controls which page of CIRAM is used for the corresponding nametable in $2000-$2FFF.

See also

• NES Mapper List by Disch
• Comprehensive NES Mapper Document by \Firebug\, information about mapper's initial state is inaccurate.