Add interrupts enabled flag to spec, add interrupt return actions to spec

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

vm.md

@@ -43,6 +43,7 @@ CPU flags are addressed by bit index, going from right to left.
 
 * `00` - Halt flag
 * `01` - Compare flag
+* `02` - Enable interrupts
 
 ### Flag ideas
 
@@ -269,6 +270,10 @@ instruction, supplying the index of the interrupt to invoke. Hardware interrupts
 directly by a hardware event, e.g. a keypress. Hardware and software interrupts are treated equally
 in the CPU, and as such, they are all maskable.
 
+An interrupt may be masked in two ways: either through its entry in the IVT, or through the "enable
+interrupts" CPU flag. If the "enabled" bit in the IVT is not set, that interrupt will not be handled
+when it is invoked. If the "enable interrupts" CPU flag is not set, *no* interrupts will be handled.
+
 ## Interrupt vector table
 
 Interrupts are defined by the IVT register. The address stored in the IVT register must be a
@@ -305,7 +310,17 @@ Before an interrupt handler is called, these actions occur:
 * Push the R0 register
 * Push the R1 register
 
-Interrupt handlers must be exited using the `iret` instruction.
+Interrupt handlers must be exited using the `iret` instruction. When an interrupt call is exited,
+the above actions occur in reverse:
+
+* Update the stack pointer to the current frame pointer + 48
+* Pop the old R1 value
+* Pop the old R0 value
+* Pop the old STATUS value
+* Pop the old FLAGS value
+* Pop the IP of the next instruction
+* Pop the old stack frame
+* Restore the last 6 values in an undefined order
 
 ## Exceptions