Zilog Z08470 - Manual (Page 14)

Architectural Overview UM008007-0715
2
Z80 CPU
User Manual
CPU Register
The Z80 CPU contains 208 bits of read/write memory that are available to the program-
mer. Figure 2 shows how this memory is configured to eighteen 8-bit registers and four
16-bit registers. All Z80 CPU’s registers are implemented using static RAM. The registers
include two sets of six general-purpose registers that can be used individually as 8-bit reg-
isters or in pairs as 16-bit registers. There are also two sets of Accumulator and Flag regis-
ters and six special-purpose registers.
Special-Purpose Registers
Program Counter (PC). The program counter holds the 16-bit address of the current
instruction being fetched from memory. The Program Counter is automatically incre-
mented after its contents are transferred to the address lines. When a program jump occurs,
the new value is automatically placed in the Program Counter, overriding the incrementer.
Stack Pointer (SP). The stack pointer holds the 16-bit address of the current top of a stack
located anywhere in external system RAM memory. The external stack memory is orga-
nized as a last-in first-out (LIFO) file. Data can be pushed onto the stack from specific
CPU registers or popped off of the stack to specific CPU registers through the execution of
PUSH and POP instructions. The data popped from the stack is always the most recent
data pushed onto it. The stack allows simple implementation of multiple level interrupts,
unlimited subroutine nesting and simplification of many types of data manipulation.
Figure 2. CPU Register Configuration
General
Purpose
Registers
Accumulator
H '
Special
Purpose
Registers Index Register
Index Register
Stack Pointer
Program Counter
Interrupt Vector
I
H L L '
D E D ' E '
B C B ' B '
A F A ' F '
Flags Accumulator Flags
Alternate Register Set Main Register Set
Memory Refresh
R
IX
IY
SP
PC