Add a few notes about numbers and arithmetic, rename Neg -> INeg

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

vm.md

@@ -6,6 +6,7 @@ This is an outline of the VM that drives this language.
 
 * Numbers may be big endian (BE) or little endian (LE) at the byte level. This guide will use LE.
 * Addresses point to single bytes.
+* Signed numbers use two's complement.
 
 | Type     | Size (bits) |
 | -        | -           |
@@ -43,23 +44,29 @@ CPU flags are addressed by bit index, going from right to left.
 
 ## Arithmetic
 
-Arithmetic instructions store their result in the first register specified.
+Arithmetic instructions store their result in the first register specified. Overflow is handled by
+wrapping around to 0.
+
 
 * Add
     * **Params**: REG1, REG2
     * `REG1 = REG1 + REG2`
+    * Unsigned addition
 * Mul
     * **Params**: REG1, REG2
     * `REG1 = REG1 * REG2`
+    * Unsigned multiplication
 * Div
     * **Params**: REG1, REG2
     * `REG1 = REG1 / REG2`
+    * Unsigned division
 * Mod
     * **Params**: REG1, REG2
     * `REG1 = REG1 % REG2` (exact semantics TBD)
-* Neg
+* INeg
     * **Params**: REG1
     * `REG1 = REG1 * -1`
+    * Signed negative
 * And
     * **Params**: REG1, REG2
     * `REG1 = REG1 & REG2`
@@ -76,6 +83,11 @@ Arithmetic instructions store their result in the first register specified.
     * **Params**: REG1, REG2
     * `REG1 = REG1 >> REG2`
 
+### TODO
+
+* Add signed instructions (iadd, imul, etc)
+* Overflow flag?
+
 ## Control flow
 
 * CmpEq