Programming MMC1: Difference between revisions
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(Then, to switch banks) |
(see also how to load CHR RAM) |
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'''MMC1''' was Nintendo's first ASIC mapper for the NES. | |||
== Quick setup for UNROM style == | == Quick setup for UNROM style == | ||
If you are using SGROM or SNROM to provide an environment similar to [[Programming UNROM|UNROM]], with 8 KB of CHR RAM, a fixed PRG ROM bank at $C000, and a 16 KB switchable PRG ROM bank at $8000, do this in your init code: | If you are using the [[SGROM]] or [[SNROM]] board to provide an environment similar to [[Programming UNROM|UNROM]], with 8 KB of CHR RAM, a fixed PRG ROM bank at $C000, and a 16 KB switchable PRG ROM bank at $8000, do this in your init code: | ||
<pre> | <pre> | ||
lda #$80 ; reset the mapper | lda #$80 ; reset the mapper | ||
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Later revisions guarantee that the fixed bank is loaded at power on. | Later revisions guarantee that the fixed bank is loaded at power on. | ||
Then to switch banks, load the bank number into A and call this subroutine: | Then to switch PRG ROM banks, load the bank number into A and call this subroutine: | ||
<pre> | <pre> | ||
mmc1_load_prg_bank: | mmc1_load_prg_bank: | ||
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rts | rts | ||
</pre> | </pre> | ||
Most games seem to switch to another bank before they [[CHR-ROM vs CHR-RAM#Switching to CHR RAM|load CHR RAM]]. | |||
== See also == | |||
* [[MMC1]] technical reference | |||
Revision as of 14:58, 6 June 2010
MMC1 was Nintendo's first ASIC mapper for the NES.
Quick setup for UNROM style
If you are using the SGROM or SNROM board to provide an environment similar to UNROM, with 8 KB of CHR RAM, a fixed PRG ROM bank at $C000, and a 16 KB switchable PRG ROM bank at $8000, do this in your init code:
lda #$80 ; reset the mapper sta $E000 lda #$0E ; vertical mirroring, fixed $C000, 8 KB CHR pages sta $8000 ; (use $0F instead for horizontal mirroring) lsr a sta $8000 lsr a sta $8000 lsr a sta $8000 lsr a sta $8000
Very early revisions of the MMC1 IC might power up in a mode other than fixed-$C000, requiring that the vectors and the start of the init code be placed in all banks, much as in BxROM or AxROM or GxROM. Later revisions guarantee that the fixed bank is loaded at power on.
Then to switch PRG ROM banks, load the bank number into A and call this subroutine:
mmc1_load_prg_bank: sta $E000 lsr a sta $E000 lsr a sta $E000 lsr a sta $E000 lsr a sta $E000 rts
Most games seem to switch to another bank before they load CHR RAM.
See also
- MMC1 technical reference
