drones

Drones

At some point this will turn into a video game. The concept is robot combat, but instead of controlling the game directly players provide code for a VM that controls their robot. Currently, only the VM is working.

VM Documentation

The Drones VM is a strictly 16-bit machine: all of its registers are 16 bits, and its memory is addressed in 16-bit words (not bytes!).

Registers

• ip - Instruction pointer. Tells the machine where to fetch the next opcode from.
• bp - Base pointer. Points to the return address for the current activation record.
• sp - Stack pointer. Points one element below the top of the stack (the stack grown downwards).
• a - Accumulator. Opcodes that compute a result will generally store their output here.
• b - A second register, used as the source for some which operate on multiple values.
• p - Pointer register, used for dynamic memory loading operations.
• i - Index register, used for indexed addressing.

Buses

Buses are the VM's means of communication with the outside world. They're a set of 16-bit words which can be directly accessed by the code running the VM. From inside the VM you can write and read them with the wb and rb instructions. From outside the VM, you can directly access the Buses member of the VM struct, which is a slice of uint16.

Operation

The initial state of the machine puts sp and bp at the highest address (the stack grown downwards from the top of memory) and every other register at 0. In a clock cycle, the machine loads the instruction at address ip and an argument from address ip + 1, increments ip by 2, and then executes the opcode with the argument. Note that all opcodes take up 2 16-bit words in memory, for those which don't use arguments the content of the second word is irrelevant. A clever programmer or compiler may re-use those spaces in memory. Execution continues in this way for as long as the machine runs.

Stack Frame

There are two special opcodes for dealing with function calls, call and ret. When call is executed, the VM jumps to its argument address. At the same time, it pushes the address of the instruction that would have been executed next and the current base pointer to the stack, and sets bp to point to the return address that it just pushed. When ret is executed, it restores the previous base pointer and jumps to the return address. This has the effect of dropping anything from the stack that was pushed during the function's execution (so local variables can be safely pushed to the stack). Everything pushed before beginning function execution will be just above the base pointer, so you can access arguments pushed onto the stack by the calling function using indexed addressing.

Instruction Set

The instruction set of the drones VM is extremely minimal. Most every operation operates on a and possibly b and stores its result in a. Loading from constants or memory can only be done into a, and from there a set of opcodes is available to swap values into other registers of interest. The one exception to this rule is the lbp instruction, which loads the base pointer into the p register.

Jumps

• jmp - Jumps to the argument address.
• jz - Jumps to the argument address only if a == 0.
• jnz - Jumps to the argument address only if a != 0.

Swaps

• sab - Swaps the a and b registers.
• sap - Swaps the a and p registers.
• sai - Swaps the a and i registers.

Loads

• lbp - Loads the b register into p.
• ldc - Loads a constant argument into a.
• ldm - Loads from memory at the argument address into a.
• ldp - Loads from memory at the address in p into a.
• ldi - Loads from memory at the address in (p + i) into a.

Stack Manipulation

• push - Pushes the content of a onto the stack.
• pop - Pops the top of the stack into a.

Buses

• rb - Reads from the bus number in the argument into a.
• wb - Writes from a into the bus number in the argument.

Function Calls

• call - Jumps to the address in the argument preparing a new stack frame for a function call. Does not automatically preserve registers, push them manually if you want to save them.
• ret - Returns from a function called with call.

Arithmetic

• add - a <- a + b
• sub - a <- a - b
• mul - a <- a * b
• div - a <- a / b, b <- a % b
• sdiv - a <- a / b, b <- a % b (signed)

Bitwise Operations

• and - a <- a & b
• or - a <- a | b
• xor - a <- a ^ b
• shl - a <- a << b
• shr - a <- a >> b
• not - a <- ~a

Comparisons

• lt - a <- a < b ? 0xffff : 0
• lts - a <- a < b ? 0xffff : 0 (signed)
• le - a <- a <= b ? 0xffff : 0
• les - a <- a <= b ? 0xffff : 0 (signed)
• gt - a <- a > b ? 0xffff : 0
• gts - a <- a > b ? 0xffff : 0 (signed)
• ge - a <- a >= b ? 0xffff : 0
• ges - a <- a >= b ? 0xffff : 0 (signed)
• eq - a <- a == b ? 0xffff : 0
• neq - a <- a != b ? 0xfff : 0