Zilog Z80180 - Manual (Page 85)

Z8018x
Family MPU User Manual
70
UM005003-0703
TRAP Interrupt
The Z8X180 generates a non-maskable (not affected by the state of IEF1)
TRAP interrupt when an undefined Op Code fetch occurs. This feature
can be used to increase software reliability, implement an extended
instruction set, or both. TRAP may occur during Op Code fetch cycles
and also if an undefined Op Code is fetched during the interrupt
acknowledge cycle for INT0 when Mode 0 is used.
When a TRAP interrupt occurs the Z8X180 operates as follows:
1. The TRAP bit in the Interrupt TRAP/Control (ITC) register is set to
1
.
2. The current PC (Program Counter) value, reflecting location of the
undefined Op Code, is saved on the stack.
3. The Z8X180 vectors to logical address 0. Note that if logical address
0000H
is mapped to physical address
00000H
. the vector is the same
as for RESET. In this case, testing the TRAP bit in ITC reveals
whether the restart at physical address
00000H
was caused by
RESET or TRAP.
The state of the UFO (Undefined Fetch Object) bit in ITC allows TRAP
manipulation software to correctly adjust the stacked PC, depending on
whether the second or third byte of the Op Code generated the TRAP. If
UFO is
0
, the starting address of the invalid instruction is equal to the
DI 0 0
LD A, I not affected not affected Transfers the contents of IEF1 to
P/V
LID A, R not affected not affected Transfers the contents of IEF1 to
P/V
Table 8. State of IEF1 and IEF2 (Continued)
CPU
Operation
IEF1 IEF2 REMARKS