All About the HuC1
The HuC1 is an MBC used by some Game Boy games which besides doing the usual MBC stuff, also provides IR comms. A lot of sources on the internet say that HuC1 is "similar to MBC1", but they don't provide a lot of detail. I took a look, and it turns out that HuC1 differs from MBC1 quite a bit.
0000-1FFF IR select 2000-3FFF ROM bank select 4000-5FFF RAM bank select 6000-7FFF Nothing? -- A000-BFFF Cart RAM or IR register
0000-1FFF IR Select (Write Only)
Most MBCs let you disable the cartridge RAM to prevent accidental writes. HuC1 doesn't do this, instead you use this register to switch the A000-BFFF region between "RAM mode" and "IR mode" (described below). Write 0x0E to switch to IR mode, and anything else to switch to RAM mode. Nevertheless some HuC1 games write 0x0A and 0x00 to this region as if it would enable/disable cart RAM.
2000-3FFF ROM Bank Number (Write Only)
HuC1 can accept a bank number of at least 6 bits here.
4000-5FFF RAM Bank Select (Write Only)
HuC1 can accept a bank number of at least 2 bits here.
6000-7FFF Nothing? (Write Only)
Writes to this region seem to have no effect. Even so, some games do write to this region, as if it had the same effect as on MBC1. I believe an emulator can safely ignore these writes.
A000-BFFF Cart RAM or IR register (Read/Write)
When in "IR mode" (wrote 0x0E to 0x0000), the IR register is visible here. Write to this region to control the IR transmitter. 0x01 turns it on, 0x00 turns it off. Read from this region to see either 0xC1 (saw light) or 0xC0 (did not see light). When in "RAM mode" (wrote something other than 0x0E to 0x0000) this region behaves like normal cart RAM.