Virtual Machine C++

A virtual machine / CPU emulator written in C++. It contains a VM, assembler, and debugger.

CPU

The CPU contains four general purpose registers, named AX, BX, CX, and DX. There is a stack pointer SP which indicates the top of the stack. There is a base pointer BP which is the bottom of the stack. The stack grows upwards, to lower memory. Finally there is the instruction pointer IP.

Name	Value	Bits	Description
AX	1	16	Register A
BX	2	16	Register B
CX	4	16	Register C
DX	8	16	Register D
SP	16	16	Stack pointer
SB	32	16	Stack base
BP	64	16	Base pointer (consider replacing with flags)
IP	128	16	Instruction pointer

The following flags are used in the virtual machine.

Name	Value	Description
C	1	Carry
Z	2	Zero
I	4	Interrupt
B	8	Break
O	16	Overflow
E	32	Equal
L	64	Less than
G	128	Greater than

Memory

Memory has the following layout:

• In the constructor of the virtual machine the amount of memory is currently set to 8kb.
• The address range is 0x0000 - 0xffff.
• The VGA is mapped to address 0x2000 in memory.
  • Size is set at 76x20 so 1520 bytes, range 0x2000-0x25f0.
• At the moment, a ROM is loaded at address 0x0000.
• The stack is located at the end of memory.

Instructions

The virtual machine support the following opcodes.

Including files

Before assembling, other assembly files should be included. They are annotated with the @include directive.

%include("file.asm)

Debugger

• Press F5 to halt.
• Enter run to start (shorthand: r).
• Enter reset to reset.
• Enter step to execute one instruction (shorthand: s).
• Enter exit to exit.
• Enter mem addr to change memory view (shorthand: m).
• Enter set_reg reg val to set a register.
• Enter set_mem addr val to set a memory address.
• Enter cursor addr to set the write cursor at that address (shorthand: c).
• Enter assemble opcode arg1 arg2 (arg3) to assemble those instruction and write them to the write cursor addres.
• Enter load file to load a binary file at the write cursor.
• Enter reload to reload the file at the write cursor.
• Enter b addr to toggle a breakpoint (not added yet).

Todo

• Write an assembler in Python.
  • Allow including of asm files with %include("file.asm").
  • Implement labels for reference, defined with label:, and address is placed for label.
• Generate better disassembler argument formatting.
• Load a binary from a file.
• Debugger command: load_rom, to load a binary into the VM.
• Debugger command: reload, to reload the last load rom, for easier dev
• Debugger command: reg = value, set the value of a register.
• Debugger command: [mem] = value, set the value of a memory address.
• Debugger breakpoints.
• Expand the instruction set:
  • Arithmetic: ADD, SUB, MUL, DIV.
  • Logical operators: AND, OR, NOT, XOR, SHL, SHR.
  • Stack: POP, PUSH.
  • Functions: CALL, RET, but these can be rewritten in terms of POP, PUSH, and JMP.
• Interrupts:
  • Keyboard input?