README
¶
Simple Virtual Computer
Currently under development; this is a work in progress.
A 16-bit virtual machine written in Go.
It uses word-based memory, where each memory address maps to a word-length value. This means that while there are only 65,536 memory addresses, it technically has 128K of memory since each address points to a 16-bit value instead of a byte. The reasoning for this is simple: simplicity. Most modern computer systems use bytes because they can be more flexible and efficient. Those are not the goals of this project.
It implements a "VGA text mode" that reads the contents of memory, using 2,000 contiguous words (which is interpreted as a 80x25 character display). It translates the encoded VGA text colors into ANSI escape codes and prints the colorized ASCII text.
This repository contains the virtual machine, an assembler to compile programs for it, and a debugger for those programs.
Instruction Set
The instruction set resembles a RISC (Reduced Instruction Set Computer) architecture.
In the opcode, r represents the number of a CPU register that is packed into the word which contains the opcode to save memory.
| Opcode | Name | Operands | Description |
|---|---|---|---|
0x0000 |
nop |
Does nothing. | |
0x01rr |
cop |
reg to copy to reg to copy from |
Copies the value from one register to another. |
0x020r |
cpl |
reg value |
Copies a literal value to a register. |
0x03rr |
str |
reg holding addr reg holding value |
Stores the value from a register into memory at the address held in another register. |
0x04rr |
ldr |
reg to load to reg holding addr |
Loads the value from memory at the address held in one register into another register. |
0x050r |
add |
reg |
Adds the value held in a register to the accumulator. |
0x060r |
sub |
reg |
Subtracts the value held in a register from the accumulator. |
0x070r |
twc |
reg |
Sets a register to the two's complement of the value it holds. |
0x080r |
inc |
reg |
Increases the value held in a register by one. |
0x090r |
dec |
reg |
Decreases the value held in a register by one. |
0x0A0r |
mul |
reg |
Multiplies the accumulator with the value held in a register. |
0x0B0r |
div |
reg |
Divides the accumulator by the value held in a register, storing the modulus in the ex register. |
0x0C0r |
dcv |
reg |
Divides the accumulator by the value held in a register, taking into account two's complement negative numbers, storing the modulus in the ex register. |
0x0D0r |
xor |
reg |
Performs the "xor" operation on the accumulator with the value held in a register. |
0x0E0r |
and |
reg |
Performs the bitwise "and" operation on the accumulator with the value held in a register. |
0x0F0r |
orr |
reg |
Performs the bitwise "or" operation on the accumulator with the value held in a register. |
0x100r |
not |
reg |
Inverts the value held in a register. |
0x110r |
shr |
reg value |
Shifts the value held in a register to the right by a specified value. |
0x120r |
shl |
reg value |
Shifts the value held in a register to the left by a specified value. |
0x1300 |
vga |
Prints the VGA text buffer to the screen. | |
0x140r |
psh |
reg |
Decreases the stack pointer and sets the top value of the stack to the value held in a register. |
0x150r |
pop |
reg |
Stores the top value of the stack in a register and increases the stack pointer. |
0x1600 |
ret |
Pops the program counter off of the stack. | |
0x1700 |
cal |
addr |
Pushes the program counter onto the stack and sets the program counter to an address. |
0x18rr |
cmp |
reg reg |
If the values of two registers are the same, the boolean index (bi register) is set to 0xffff, else 0xfffe. |
0x1900 |
cle |
addr |
Equivalent to cal, but only executes if the bi register is set to 0xffff. |
0x1A00 |
cln |
addr |
Equivalent to cal, but only executes if the bi register is set to 0xfffe. |
0x1B00 |
gto |
addr |
Sets the program counter to an address. |
0x1C00 |
gte |
addr |
Equivalent to gto, but only executes if the bi register is set to 0xffff. |
0x1D00 |
gtn |
addr |
Equivalent to gto, but only executes if the bi register is set to 0xfffe. |
0x1E0r |
cml |
reg value |
Equivalent to cmp, but the second operand is a literal value, not a register. |
CPU Registers
There are 12 CPU registers, 8 of which are general purpose.
| Number | Alias | Purpose |
|---|---|---|
0x0 |
ra |
General purpose; used for whatever you desire. |
0x1 |
rb |
General purpose. |
0x2 |
rc |
General purpose. |
0x3 |
rd |
General purpose. |
0x4 |
re |
General purpose. |
0x5 |
rf |
General purpose. |
0x6 |
rg |
General purpose. |
0x7 |
rh |
General purpose. |
0x8 |
ex |
Extra: holds extra arithmetic output values, used for register expansions ((value) syntax). |
0x9 |
ac |
Accumulator: holds the output of most arithmetic operations. |
0xa |
sp |
Stack pointer: holds the address of the top location in memory of the stack. |
0xb |
pc |
Program counter: holds the address of the next instruction in memory to be executed. |
0xc |
bi |
Boolean index: set to 0xffff if the last cmp was equal, else 0xfffe. |
The Simple Virtual Assembler
The assembler reads a rudimentary assembly language and outputs a binary format called "svb".
See the sva directory for documentation on writing in the assembly language and using the assembler.
See the svd directory for using the debugger and the asm directory for some example programs.
Memory
Sections:
- VGA text buffer:
0x00-0x7d0 - Stack:
0x7d1-0x8ff - Program (varies in size):
0x900-0xX - Heap (everything else):
0xX+1-0xffff
Before the CPU starts execution, a few things are done in memory:
- The value
0xffffis pushed onto the stack. It will be pulled off with the "main" subroutine'sretinstruction. When the program counter is set to0xffffthe virtual machine will stop. - The command-line arguments are loaded into the heap. (The heap is just all of the memory that doesn't have a specific purpose.)
- The word at
0xfffdis set to the number of command-line arguments. - The word at
0xfffeis set to the size of the command-line arguments (number of characters + null terminators). - The word at
0xffffis set to the address of the start of the heap.
To Do
- More example programs and documentation.
- Better tests (ones that actually exist).
- Keyboard input.
- Virtual drive with simple filesystem.
License
The contents of this repository are made available under the Unlicense, a public domain equivalent license.
See the LICENSE file for its text.
Documentation
¶
There is no documentation for this package.
Source Files
Directories
¶
| Path | Synopsis |
|---|---|
|
Package cpu implements structs and functions for representing and operating the virtual CPU.
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Package cpu implements structs and functions for representing and operating the virtual CPU. |
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Package dat contains information about CPU opcodes and registers.
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Package dat contains information about CPU opcodes and registers. |
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Package mem implements memory devices.
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Package mem implements memory devices. |
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Package svb defines interfaces for parsing and creating Simple Virtual Binary format files.
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Package svb defines interfaces for parsing and creating Simple Virtual Binary format files. |
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Package util contains utility functions.
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Package util contains utility functions. |
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Package vga implements video devices which can show output.
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Package vga implements video devices which can show output. |