From 6fb37b2588d69609097109c0dce88fba46087618 Mon Sep 17 00:00:00 2001
From: nils <48135649+Nilstrieb@users.noreply.github.com>
Date: Mon, 20 Jun 2022 14:58:16 +0200
Subject: [PATCH] spec

---
 README.md        | 97 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/interpret.rs |  5 ++-
 2 files changed, 101 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 0f439b7..067e8c3 100644
--- a/README.md
+++ b/README.md
@@ -16,3 +16,100 @@ jmp false
 
 exit:
 ```
+
+# Specification
+
+The crapderive virtual machine has 16 registers, `r0` to `r15`.
+
+It has 100MB (1024^3) bytes of memory, addressed by the addresses `0x0` to `0x40000000` (not included).
+
+The machine word (register) size is 64 bit.
+
+Arithmetic is defined as wrappings, and division by zero traps (cannot be caught).
+
+The VM has a `flag` that is used in some operations.
+
+## Definitions
+
+### Place
+
+A place is a location in the VM. A place can either be a register (`r0`-`r15`)
+or a memory address (`[X]` where `X` is an `index`).
+
+### Index
+
+An index is used to dereference memory and can be one of the following:
+
+* A register
+* A literal number
+* A symbol (TODO)
+
+### Value
+
+A value is a value that can be obtained from the VM. A value can either be a place, a literal number or a symbol.
+
+### Location
+
+A location is a location in the VM code. It must contain a symbol with the label to redirect execution to.
+
+## Instructions
+
+### Mov
+
+`mov A, B`
+
+Mov moves the `value` `B` to the `place` `A`.
+
+### Add
+
+`add A, B`
+
+Add adds the `value` `B` to the `place` `A`.
+The result is stored in the `place` `A`.
+
+### Sub
+
+`sub A, B`
+
+Sub subtracts the `value` `B` from* the `place` `A`.
+The result is stored in the `place` `A`.
+
+### Mul
+
+`mul A, B`
+
+Mul multiplies the `place` `A` with the `value` `B`.
+The result is stored in the `place` `A`.
+
+### Div
+
+`div A, B`
+
+Div divides the `place` `A` by the `value` `B`.
+The result is stored in the `place` `A`.
+
+### Cmp
+
+`cmp A, B`
+
+Cmp compares the `value` `A` with the `value` `B`.
+If they are equal, the `flag` is set, otherwise the `flag` is cleared.
+
+### Jmp
+
+`jmp L`
+
+Jmp jumps to the `location` `L`. The next instruction executed will be the instruction located at `L`.
+
+### Je
+
+`je L`
+
+Je jumps to the `location` `L` if the `flag` is set. If the jump happens,
+the next instruction executed will be the instruction located at `L`.
+
+### Label
+`<IDENT>:`
+
+A label is an identifier (regex: `[a-zA-Z]\w+`) followed by a colon (`:`). Jumps (like `jmp` or `je`) will be able
+to jump to this label. A label is applied to the *next* instruction in the code.
\ No newline at end of file
diff --git a/src/interpret.rs b/src/interpret.rs
index d95a971..c90bd45 100644
--- a/src/interpret.rs
+++ b/src/interpret.rs
@@ -22,6 +22,8 @@ impl Register {
     }
 }
 
+const MEMORY_SIZE = 1024 * 1024 * 1024;
+
 struct InterpretCtx {
     memory: Vec<u8>,
     registers: [u64; 16],
@@ -151,9 +153,10 @@ impl InterpretCtx {
     }
 }
 
+
 pub fn interpret(stmts: Vec<Stmt>, spans: Vec<Span>) -> Result<()> {
     let mut ctx = InterpretCtx {
-        memory: vec![0; 100_000],
+        memory: vec![0; MEMORY_SIZE],
         registers: [0; 16],
         flag: false,
         stmts,