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| The [[Programming UOROM|UOROM]] mapper has 256kb PRG-ROM (divided into 16 16k banks) and CHR-RAM. It is very easy to use.
| | #REDIRECT [[Programming UNROM]] |
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| == iNES Header ==
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| Here is an iNES header for the [[Programming UOROM|UOROM]] mapper.
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| .segment "HEADER"
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| .byte "NES", $1A
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| .byte $10 ;UOROM has 16 16k banks
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| .byte $00 ;UOROM uses CHR RAM, so no CHR ROM
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| .byte $20, $00 ;UOROM is Mapper 2
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| .byte $00 ;UOROM has no PRG RAM
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| == Bankswitching ==
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| [[Programming UOROM|UOROM]] has 16 16k banks. One of these banks is fixed at $C000-$FFFF. The other fifteen (numbered 0-15, or $00-$0E) are switchable at $8000-$BFFF. | |
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| Switching banks requires a write to $8000-$FFFF. Bits 0-3 of the byte written to $8000-$FFFF will select the bank. When writing to $8000-$FFFF, the value you are writing must match the value located at the destination address in ROM (see [[Bus conflict]]). One way to ensure this is to have a bankswitch lookup table. You can read from this table and then immediately write that value back to the table.
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| .segment "BANKTABLE"
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| banktable:
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| .byte $00, $01, $02, $03, $04, $05, $06, $07 ;write to this table to switch banks.
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| .byte $08, $09, $0A, $0B, $0C, $0D, $0E
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| .segment "ZP": zeropage
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| current_bank: .res 1
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| .segment "CODE"
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| bankswitch:
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| lda banktable, y ;read a byte from the banktable
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| sta banktable, y ;and write it back, switching banks
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| sta current_bank ;store the current bank in RAM
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| rts
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| The lookup table and the bankswitching subroutine should be located in the fixed bank, so that they are always available. It is common to stick the lookup table at $FFEB-$FFF9, immediately before the nmi/reset/irq vectors (at $FFFA-FFFF).
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| With the lookup table and bankswitching subroutine in place, switching banks is as easy as this:
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| ldy #$02
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| jsr bankswitch ;switch to bank 2
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| == Related articles ==
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| [[UxROM|UxROM technical reference]]
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