Minor disassembler extension: execute instructions

[larsbrinkhoff]

When decoding instruction and syscalls, might as well add the option to execute them as well.

[larsbrinkhoff]

Idea.

Decode instructions to three function pointers:

1. Fetch operands. Cases:
   • None, e.g. JRST.
   • Just memory, e.g. SKIP.
   • Both AC and memory, e.g. ADDI.
   • Just AC, e.g. ...?
2. Do operation.
3. Write back results. Again, a few different cases.

Store these in an array shadowing main memory. Initialize all pointers to decode_instruction. Also reset to this whenever a memory location is written.

Main instruction loop:

1. Fetch instruction word.
2. EA calculation.
3. Call the three pointers.
4. Repeat.

[larsbrinkhoff]

Pure pages can be filled with decode_page. The first intrstruction to be executed triggers translation of the whole page.

[larsbrinkhoff]

EA calculation also has some subcases:

• Just Y.
• Just X.
• Y + (X).
• Indirection: full calculation.

[larsbrinkhoff]

Possibly generate machine code:

• Call EA Calculation routine.
• Call Read Operands routine.
• Call Operation routine.
• Call Write Back routine.

Machine state registers:

• PC - program counter
• MA - EA
• MB - Memory buffer
• AR - AC
• MQ