Interrupt Control

Number	$C0
Readable	Yes
Writable	Yes
Short Description

Soft reset = 0x00

 bits 7:5 = Programmable portion of im2 vector*
 bit 4 = Reserved must be 0
 bit 3 = Enable stackless nmi response**
 bits 2:1 = Current Z80 interrupt mode 0,1,2 (read only, write ignored)
 bit 0 = Maskable interrupt mode: pulse (0) or hw im2 mode (1)

• * In hw im2 mode the interrupt vector generated is:

 bits 7:5 = nextreg 0xC0 bits 7:5
 bits 4:1 = 0  line interrupt (highest priority)
   = 1  uart0 Rx
   = 2  uart1 Rx
   = 3-10  ctc channels 0-7
   = 11 ula
   = 12 uart0 Tx
   = 13 uart1 Tx (lowest priority)
 bit 0 = 0

• * In hw im2 mode the expansion bus is the lowest priority interrupter and if no vector is supplied externally then 0xFF is generated.

• ** The return address pushed during an nmi acknowledge cycle will be written to nextreg instead of memory (the stack pointer will be decremented) and the first RETN after the acknowledge will take its return address from nextreg instead of memory (the stack pointer will be incremented). If bit 3 = 0 and in other circumstances, RETN functions normally.