ZiLOG Z80 Handbook page 24

interrupt to yet another interrupt location. All four kinds of interrupt
groupings are discussed in a later chapter in this section.
The last special-purpose register is the 7-bit Memory Refresh reg-
ister R. When external memory is made up of dynamic memories,
the R register allows automatic refreshing of this kind of semicon-
ductor memory which periodically (typically every 2 milliseconds)
needs to have every cell read or refreshed to retain its contents. The
contents of the R register are incremented by one after every in-
struction fetch and the contents are sent out along the least signifi-
cant 7 bits of the address bus while the Z-80 CPU is not accessing
memory. Every cell of external memory with a predefined configura-
tion of its address bits equal to the R register can now be refreshed
without fear of contention (simultaneous read) of the same memory
cell by the Z-80 CPU. The R register is normally not used by the
programmer.
MICROCOMPUTER COMPONENT PARTS
As in any microcomputer, the microprocessor chip itself does not
constitute the complete computer system. Fig. 2-10 shows the com-
ponent parts of a typical Z-80 system. The Z-80 microprocessor chip
z-80
MICROPROCESSOR
AND ASSOCIATED
LOGIC
CONTROL
PANEL
LOGIC
(IF ANY)
ADDRESS DATA
BUS BUS
EXTERNAL
MEMORY
(RAM, ROM,
PROM,
EPROM,
1 ETC.)
110 DEVICE
CONTROLLER
1
sP 110 DEVICE 1
2
N
F^ -- 110 DEVICE 2
110 DEVICE 3
Fig. 2-10 . Z-80 Microcomputer system component parts.
along with supporting circuitry interfaces to external memory. Con-
trol signals are passed between CPU circuitry and external memory,
memory addresses are passed along the 16-bit address bus, and data
is passed along the 8-bit address bus. External memory may be any
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