This is a BBC Micro machine code monitor ROM that fits in 4K. It supports:
- Memory dumps
- Disassembly
- Setting memory values
- Executing from a memory location
It also has its own 'language' which supports all of the above with simple command names together with break points and register value reading and setting.
The monitor supports help.
MDISS ADDRESS (<+RANGE>|) ()
This command disassembles 6502 opcodes starting at ADDRESS in memory. If no RANGE or END is specified it disassembles 8 bytes. If a RANGE preceded by + is specified it disassembles at least RANGE bytes. If END is specified it disassembles at least up to END in memory. If ROM is specified it will switch to that sideways ROM bank.
MDUMP ADDRESS (<+RANGE>|) ()
This command hex dumps data starting at ADDRESS in memory. If no RANGE or END is specified it displays 8 bytes. If a RANGE preceded by + is specified it displays at least RANGE bytes. If END is specified it displays bytes up to END in memory. If ROM is specified it will switch to that sideways ROM bank.
MGO ADDRESS
JSR to code at the specified ADDRESS
MON
This command starts the monitor as a Language.
MSET ADDRESS () ...
This command sets bytes in memory starting at the specified address. Each byte is read and set, incrementing the address after each set. You can set as many bytes as you can fit on a command line.
The monitor language supports a command line which allows monitor commands and/or * commands. The commands are single character followed by parameters. They are as follows:
D - Disassemble (Arguments as MDISS above) G - Go to address (Arguments as MGO above) M - dump Memory (Arguments as MDUMP above) Q - Quit. I.e. return to BASIC R - Display registers (See below) S - Set memory (See below)
S - Set memory
This can be used in an alternative way to MSET. If you just specify the starting address, the address and current byte value will be presented after which you may type a new byte value. Return updates the value, at which point the next address and byte value will be displayed. This is repeated until ESC is pressed.
R - Show/set the shadow register values
R on its own will display the current shadow register values. These values are set when a breakpoint occurs (See BRK) below. All of the values may be updated except the program counter by specifying the register name, equals, and value or flag bit characters, as an argument.
The monitor language supports breakpoints via the BRK instruction. Although the BRK instruction is one byte, the program counter is moved on by two bytes. Typically, in non Acorn environments, the second byte is a BRK identified. In normal usage on Acorn machines the second byte is an error code and further bytes are a zero byte terminated error string. The monitor treats error 0 as a true break point (error 0 is "Silly" in BASIC). All other values are treated as normal.
When a sequence BRK BRK is seen, execution is stopped, the registers are displayed, and a prompt is shown which allows you to return to the monitor command line or continue execution.
The example above shows a small routine which prints 0 to 9. The breakpoint is set in the loop. The user continues the first couple of loops, before stopping execution, updating the value of the accumulator, and then restarting execution.