Zilog Z08470 - Manual (Page 74)

Z80 CPU Instructions UM008007-0715
62
Z80 CPU
User Manual
If Mode 0 is set, the interrupting device can insert any instruction on the data bus and
allow the CPU to execute it. Mode 1 is a simplified mode in which the CPU automatically
executes a restart (RST) at address
0038h so that no external hardware is required (the old
Program Counter content is pushed onto the stack). Mode 2 is the most powerful because
it allows for an indirect call to any location in memory. With this mode, the CPU forms a
16-bit memory address in which the upper eight bits are the contents of Register I, and the
lower eight bits are supplied by the interrupting device. This address points to the first of
two sequential bytes in a table in which the address of the service routine is located, as
shown in Figure 41. The CPU automatically obtains the starting address and performs a
CALL instruction to this address.
Table 19. Miscellaneous CPU Control
NOP 00
HALT 76
Disable INT (EI) F3
Enable INT (EI) FB
Set INT Mode 0
IM0 ED
46
8080A mode
Set INT Mode 1
IM1 ED
56
Call to address 0038h
Set INT Mode 2
IM2 ED
5E Indirect call using Register I and B bits from
INTER device as a pointer
Figure 41. Mode 2 Interrupt Command
Pointer to Interrupt Table,
Address of Interrupt
Service Routine
Register I is Upper Address,
Peripheral Supplies
Lower Address