Sharp MZ-80B Owner's Manual page 78

MAIN REG SET
ALTERNATE REG SET
--
ACCUMULATOR
A
B
D
H
I
FLAGS ACCUMULATOR
F
A'
c
B'
E
D'
L
H
INTERRUPT
I
MEMORY
VECTOR
REFRESH
I R
INDEX REGISTER IX
INDEX REGISTER IY
STACK POINTER SP
PROGRAM COUNTER PC
FLAGS
F'
c·
E'
L '
SPECIAL
PURPOSE
REGISTERS
Z-80A CPU REGISTER CONFIGURATION
FIGURE 1.0-2
GENERAL
PURPOSE
REGISTERS
71
3. Two Index Registers {IX & IY). The two independent index registers hold a 16-bit base address that is used in
indexed addressing modes. In this mode, an index register is used as a base to point to a region in memory from
which data is to be stored or retrieved. An additional byte is included in indexed instructions to specify a dis-
placement from this base. This displacement is specified as a two's complement signed integer. This mode of
addressing greatly simplifies many types of programs, especially where tables of data are used .
4. Interrupt Page Address Register {I). The Z-80A CPU can be operated in a mode where an indirect call to any
memory location can be achieved in response to an interrupt. The I Register is used for this purpose to store the
high order 8-bits of the indirect address while the interrupting device provides the lower 8-bits of the address.
This feature allows interrupt routines to be dynamically located anywhere in memory with absolute minimal
access time to the routine.
5. Memory Refresh Register {R). The Z-80A CPU contains a memory refresh counter to enable dynamic memories
to be used with the same ease as static memories. Seven bits of this 8-bit register are automatically incremented
after each instruction fetch. The eighth bit will remain as programmed as the result of an LD R , A instruction.
The data in the refresh counter is sent out on the lower portion of the address but along with a refresh control
signal while the CPU is decoding and executing the fetched instruction. This mode of refresh is totally transparent
to the programmer and does not slow down the CPU operation. The programmer can load the R register for test-
ing purposes, but this register is normally not used by the programmer. During refresh, the contents of the I
register are placed on the upper 8 bits of the address bus.
Accumulator and Flag Registers
The CPU includes two independent 8-bit accumulators and associated 8-bit flag registers. The accumulator holds
the results of 8-bit arithmetic or logical operations while the flag register indicates specific conditions for 8 or 16-bit
operations, such as indicating whether or not the result of an operation is equal to zero. The programmer selects the
accumulator and flag pair that he wishes to work with a single exchange instruction so that he may easily work with
either pair.