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Adds documentation for SuperVM.

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Felix Queißner 2016-05-19 01:34:21 +02:00
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# SuperVM
SuperVM is a stack machine with a simple, but flexible command
set.
## The Stack
The virtual machine utilizes a stack to provide operands to instructions.
This stack stores temporary values the program is working with.
## Registers and Flags
The SuperVM virtual machine is a stack machine, but has also some control
registers that can be set with special instructions. The registers mainly
control stack access or control flow.
| Mnemonic | Register | Function |
|----------|---------------|-------------------------------------------------|
| SP | Stack Pointer | Stores the current 'top' position of the stack. |
| BP | Base Pointer | Stores the current stack frame position. |
| FG | Flag Register | Stores the state of the flags. |
### Stack Pointer
The stack pointer points to the top of the stack. Each `push` operation increases
the stack pointer by one, each `pop` operation reduces it by one.
### Base Pointer
The base pointer is a pointer that can be set to access the stack relative to it.
The indended use is to create a stack frame with the base pointer by pushing the
previous base pointer to the stack and setting the new base pointer to the current
stack pointer.
Returning a function with this mechanism is moving the stack pointer to the current
base pointer, then popping the previous base pointer from the stack.
### Flag Register
| Bit | Flag | Option |
|-----|----------|---------------------------------|
| 0 | Zero | Is set when the output is zero. |
| 1 | Negative | Is set when the MSB is set. |
## Instructions
An SuperVM instruction is composed of multiple components:
| Component | Range | Size | Function |
|-----------|-------------------|------|-------------------------------------------|
| execution | See below. | 6 | When is the instruction executed. |
| input0 | Zero/Pop/Peek/Arg | 2 | Where does input0 come from? |
| input1 | Zero/Pop | 1 | Where does input1 come from? |
| command | [8bit] | 8 | Which command is executed? |
| cmdinfo | [12bit] | 12 | Parameter value for the command. |
| flagmod | yes/no | 1 | Does this command modifies flags? |
| output | Discard/Push/Jump | 2 | What is done with the output? |
| argument | [32bit] | 32 | Some commands can take extra information |
### Execution Modes
The execution mode checks whether the instruction will be execution or not. The execution
depends on the state of the flags. An `X` means "Don't care", a `0` means the flag must be
cleared and a `1` means the flag must be set.
An instruction is only executed when all conditions are met.
| Flag | Range |
|-----------|-------|
| Zero | X/0/1 |
| Negative | X/0/1 |
### Commands
| ID | Command | Action |
|----|-------------|--------------------------------------|
| 0 | COPY | output = input0 |
| 1 | STORE | output = MEMORY[input0] = input1 |
| 2 | LOAD | output = MEMORY[input0] |
| 3 | GET | output = STACK[BP + input0] |
| 4 | SET | output = STACK[BP + input0] = input1 |
| 5 | BPGET | output = BP |
| 6 | BPSET | output = BP = input0 |
| 7 | RSTSTACK | output = SP = BP |
| 8 | | |
| 9 | | |
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| 15 | | |
## Assembler Mnemonics