Date   : Tue, 09 Mar 2004 00:41:32 -0000
From   : tom@...
Subject: CLI then SEI

Here's an interesting thing that I found out whilst tearing my hair 
out over Empire Strikes Back.

Empire Strikes Back sets the OS' System VIA IRQ mask via OSBYTE &E9 
such that the T1 flag is clear. Eventually, T1 times out, and never 
gets acknowledged. Empire Strikes Back then calls OSBYTE &7A, which 
after a bit of faffing runs this bit of code:

 F129    JSR     &F12E   ;call autoscan
 F12C    CLI             ;allow interrupts 
 F12D    SEI             ;disable interrupts

On my emulator, the moment CLI is finished, the T1 interrupt happens. 
The OS runs the IRQ routine, but doesn't acknowledge the T1 flag, and 
so the moment RTI has run (popping the status register with I clear) 
the interrupt happens again. And then again. And then again.

But on a real BBC, it seems there's some kind of pipelining-like 
behaviour going on. After a bit of playing around, I found that the 
interrupt doesn't happen until after the instruction following the 
CLI has run! So a CLI/SEI pair won't cause an infinite chain of 
interrupts, though a CLI/NOP/SEI will. (The address on the stack on 
entry to the IRQ routine is the instruction after the instruction 
after the CLI.)

Very strange.

Should I have known this already?! :)

--Tom