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### 2.6 CBM-II emulator features

The CBM-II emulator emulates several types of CBM-II models. Those models are known under different names in the USA and Europe. In the States they have been sold as `B128` and `B256`, in Europe as `CBM 610`, `CBM 620` (low-profile case) or `CBM 710` and `CBM 720` (high-profile case with monitor). In addition to that now an experimental C510 emulation is included. The C510 (also known as P500) is the little brother of the C600/700 machines. It runs at roughly 1 MHz and, surprise, it has a VIC-II instead of the CRTC. Otherwise the different line of computers are very similar.

These computers are prepared to take a coprocessor board with an 8088 or Z80 CPU. Indeed there are models `CBM 630` and `CBM 730` that supposedly had those processors. However these models are not emulated.

The basic difference is the amount of RAM these machines have been supplied with. The `B128` and the `CBM *10` models had 128KiB RAM, the others 256KiB. This implies some banking scheme, as the 6502 can only address 64KiB. And indeed those machines use a 6509, that can address 1 MiB of RAM. It has 2 registers at addresses 0 and 1. The indirect bank register at address 1 determines the bank (0-15) where the opcodes `LDA (zp),Y` and `STA (zp),Y` take the data from. The exec bank register at address 0 determines the bank where all other read and write addresses take place.

The business line machines (C6xx/7xx) have the RAM in banks 1-2, resp. 1-4. All available banks are used for BASIC, where program code is separated from all variables, resp. from normal variables, strings and arrays that are distributed over other banks. The C510 instead has RAM in banks 0 and 1, and uses bank 1 for program and all variables. Bank 0, though, can be accessed by the VIC-II to display graphics.

Many models have been expanded to more than the built-in memory. In fact some machines have been expanded to the full 1MiB. Bank 15 is used as system bank, with only little RAM, and lots of expansion cartridge ROM area, the I/O and the kernal/basic ROMs. Some models have been modified to map RAM into the expansion ROM area. Those modifications can be emulated as well.

The different settings are described in see Changing CBM-II model.

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