loop.md

LOOP

Command LOOP has actual name: "декремент и пропуск"

Here you will find at least 3 ways how to break the loop command

Direct load

When used with direct load like:

loop #75

Loop will override itself with direct load content decremented and transfer control dependent on payload. Thus if positive number (e.g. 0x75) is provided as direct load, loop will transfer control to next command as if value in memory cell was positive. And if payload is negative (e.g. 0xF5) loop will behave like with negative value in memory cell i.e. jump over next command.

(Positive payload) Before execution:

-> 8F75
   0700
   0100

After:

   0074
-> 0700
   0100

(Negative payload) Before execution:

-> 8FF5
   0700
   0100

After execution:

   FFF5 ; sign is bit extended, therefore you see FFF5 instead of 00F5
   0700
-> 0100

Note: other addressing modes should work as expected

Indirect autoIncrement

Normal

This addressing mode will not break loop command if used as below:

org 0x10
word 0x5           ; <- operand
POINTER: word 0x10 ; <- address
loop (POINTER)+    ; <- autoIncrement
inc
hlt

1. Increment address and place it into Data Register

2. Store Data Register content (i.e. incremented address) to POINTER (0x10 -> 0x11)

3. Decrement address stored in Data Register (DR for short) back (0x11 -> 0x10). Notice, that DR has not been cleared cleared after address increment, so address remains in register

4. loop decrements operand 0x5 (located at address stored in DR i.e. 0x10). (0x5 -> 0x4)

Infinite loop DOES NOT happen because loop performs decrement on operand and NOT on the address.

The WTF

But wait! What... What if address and operand were the same essence. So... Here it is:

org 0x10
POINTER: word 0x10 ; notice pointer points to itself
LP: loop (POINTER)+
jump LP
hlt

Please! Don't run it as this will break universe to irrecoverable state!

So that's how we trigger infinite loop.

0x10 -> 0x11 -> 0x10 and so on.

Notice that increment happens first, because address increment belongs to address fetch phase. In turn, operand decrement happens during Execution phase. Address fetch precedes Execution.

Overflow

If value in memory cell is equal to 0x8000 or 0x8001 then loop will NOT jump! Due to how loop performs checks, Internal overflow occurs with these values.