Add more notes, some other TODOs, STATUS register

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

vm.md

@@ -23,7 +23,8 @@ CPU registers are addressed by a value between 0-63 (6 bits). All registers are
 * SP - Stack pointer
 * FP - Frame pointer
 * FLAGS - CPU flags
-* (10 unused registers)
+* (9 unused registers)
+* STATUS - Generic status code
 * R0-R49
 
 ## CPU Flags
@@ -82,10 +83,12 @@ wrapping around to 0.
 * Shr
     * **Params**: REG1, REG2
     * `REG1 = REG1 >> REG2`
+    * Does not sign extend
 
 ### TODO
 
 * Add signed instructions (iadd, imul, etc)
+* Sign-extending SHR
 * Overflow flag?
 
 ## Control flow
@@ -99,6 +102,7 @@ wrapping around to 0.
             FLAGS[1] = 0;
         }
     ```
+    * Sets the COMPARE flag to 1 if REG1 == REG2
 * CmpLt
     * **Params**: REG1, REG2
     * ```
@@ -108,20 +112,27 @@ wrapping around to 0.
             FLAGS[1] = 0;
         }
     ```
+    * Sets the COMPARE flag to 1 if REG1 < REG2
 * Jz
     * **Params**: REG1
     * ```
-        if FLAGS[0] == 0 {
+        if FLAGS[1] == 0 {
             IP = REG1;
         }
     ```
+    * Jumps to the address in REG1 if COMPARE flag is 0.
 * Jnz
     * **Params**: REG1
     * ```
-        if FLAGS[0] != 0 {
+        if FLAGS[1] != 0 {
             IP = REG1;
         }
     ```
+    * Jumps to the address in REG1 if COMPARE flag is 1.
+* Halt
+    * **Params**: (none)
+    * `FLAGS[0] = 1`
+    * Halts the machine
 
 ## Data movement
 
@@ -130,17 +141,41 @@ wrapping around to 0.
     * ```
     REG1 = MEM[REG2];
     ```
+    * Sets REG1 to the value at the memory address in REG2.
 * Store
     * **Params**: REG1, REG2
     * ```
     MEM[REG2] = REG1;
     ```
+    * Sets the value at the memory address in REG2 to the value in REG1.
 * StoreImm32
     * **Params**: REG1, IMM_32
     * `REG1 = IMM_32`
+    * Sets REG1 to the specified 32-bit number.
 * MemCopy
     * **Params**: REG1, REG2
     * `MEM[REG1] = MEM[REG2]`
+    * Copies the value at the memory address in REG2 to the memory address in REG1.
 * RegCopy
     * **Params**: REG1, REG2
     * `REG1 = REG2`
+    * Copies the value in REG2 into REG1.
+
+## Other instructions TODO
+
+* Call
+    * Takes address and number of bytes on the stack that are for args(?)
+    * Updates SP, FP, IP, storing previous values starting at the new FP
+* Ret
+    * Uses FP to determine previous SP, FP, and IP and restores them
+* Push
+* Pop
+* More immediate stores?
+
+# General TODO
+
+* Interrupts
+* MMIO regions
+* Execution pipeline
+    * Helps to define when certain side effects happen (e.g. when the IP increments)
+* Paging?