ZiLOG Z8 Series

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CPU

User Manual

UM001604-0108 Address Space

Registers can be accessed as either 8-bit or 16-bit registers using Direct, Indirect, or

Indexed Addressing. All 236 general-purpose registers can be referenced or modified by

any instruction that accesses an 8-bit register, without the requirement for special instruc-

tions. Registers accessed as 16 bits are treated as even-odd register pairs (there are 118

valid pairs). In this case, the data’s most significant byte (MSB) is stored in the even num-

bered register, while the least significant byte (LSB) goes into the next higher odd num-

bered register. See Figure 2.

By using a logical instruction and a mask, individual bits within registers can be accessed

for bit set, bit clear, bit complement, or bit test operations. For example, the instruction

AND R15, MASK performs a bit clear operation, Figure 3 on page 7 displays this

example.

F4 T0 Timer/Counter 0

F3 PRE1 T1 Prescaler

F2 T1 Timer/Counter 1

F1 TMR Timer Mode

F0 SIO Serial I/O

EF R239

General-Purpose Registers (GPR)

04 R4

03 P3 Port 3

02 P2 Port 2

01 P1 Port 1

00 P0 Port 0

Figure 2. 16-Bit Register Addressing

Table 2. Z8 Standard Register File (Continued)

Hex Address

Identifier Register Description

MSB

LSB

Rn Rn+1

n = Even Address

ZiLOG Z8 Series - Page 13