ZiLOG Z80 Handbook page 85

RETI and RETN are two special instructions that provide for
special actions for returning from an external maskable interrupt
(RETI) and nonmaskable interrupt (RETN). They will be dis-
cussed in Chapter 7.
RST P is also an instruction present in the 8080 and 8008. It is
used for two operations. The primary operation is as an instruction
that an interrupting device "jams" onto the data bus to effect a
vectored interrupt. The subordinate function is to allow a special
call to one of eight page 0 locations. The interrupt functions will
be discussed in Chapter 7. When the RST P is used to call a page 0
location, the instruction acts as any unconditional call. The jump is
made to one of eight page 0 locations based on the T field of the
RST P as shown in Fig. 5-22.
INPUT AND OUTPUT GROUP
The last grouping of Z-80 instructions (Table 5-10) is the Input
and Output group. The instructions in this group allow for transfer
of 8-bit bytes of data to and from CPU registers A, B, C, D, E, H, or
L with any of 256 possible I/O device addresses specified in the in-
struction. In addition, block transfers similar to the block transfers
in the previous block transfer group can be implemented. Up to 64K
bytes may be transferred semi-automatically or automatically be-
tween an I/O device and a memory block by using the I/O block
transfer instructions (INI, INIR, IND, INDR, OUTI, OTIR, OUTD,
and OTDR). The I/O instructions will be covered in detail in Chap-
ter 15 of this section.
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