6502Em

6502Em's Emulation

The processor emulated is the 65C12 as in the Master 128, with the following exceptions:

• Opcodes &07, &0B, &4B, &87, &DC emulate the respective 6502 undocumented opcodes
• Opcode &07 when in Sideways ROM issues a SWI under RISC OS (see 'Opcode07')
• The timing of opcodes when in BCD mode is as for the original 6502, not the 65C12
• The flags are not fully emulated in BCD mode
• A few of the opcode timings may not quite be correct for 1MHz memory accesses
• Opcodes &03 and &23 are used for internal purposes by the emulator
• The JMP (&xxFF) bug is emulated for the BBC and Electron modes

NB "Unused" Opcodes are defined to be NOPs

NB2 The extra Rockwell opcodes are not supported

The 6845 CRTC

Registers 0,1,6,7,9,10,11,12,13,14,15 are fully emulated

Registers 3,5 and 9 are partially emulated

The Video ULA

Fully emulated, except for "mode 8"

Gap modes are displayed without gaps

The 6850 ACIA & Serial ULA

Not Emulated

The Paged ROM Select Register (ROMSEL)

Fully Emulated

Supports ROM or RAM in any of the 16 banks

The 6522 VIAs

The VIAs are fully emulated except for the shift register

The System VIA

Interrupts supported from 6845, keyboard and the two timers

Interrupts from 7002 ADC and light pen not supported

Keyboard, sound and the screen wrap address supported

Caps Lock LED emulated

Shift Lock LED not supported

The User Via

Interrupts supported from the 2 timers and printer.

User port emulated using OS_Bytes 150,151 for offsets &60 and &62

NB pins CB1,CB2 and corresponding interrupts not emulated

The 7002 ADC

Returns the mouse's position on both channels unless a joystick module is loaded that conforms to Acorn's Joystick SWI specifications.

Econet & Disc Hardware

Not Emulated

Tube Hardware

Not Emulated