ZiLOG Z8 Technical Manual

Address Spaces
By using logical instructions and a mask, indivi-
dual bits within registers can be accessed for bit
set, bit clear, bit complement, or bit test opera-
tions. For example, the instruction AND R, MASK
performs a bit clear operation.
When instructions are executed, registers are read
when defined as sources and written when defined
as destinations. All general-purpose registers
function as accumulators, address pointers, index
registers, stack areas, or scratchpad memory.
Z8 instruct ions can access 8-bit registers and
register pairs (16-bit) using either 4-bit or
8-bit address fields. With 4~bit addressing, the
register file is logically divided into 9 groups
of 16 working registers as shown in Figure 3-3. A
Register Pointer (one of the control registers)
contains the base address of the act i ve working
register group.
When accessing one of the working registers, the
4-bit address is concatenated with the upper four
bits of the Register Pointer, thus forming an
8-bit address. Figure 3-4 illustrates this opera-
tion. Since working registers are typically
specified by short format instructions, there are
fewer bytes of code needed, which reduces execu-
tion time. In addition, when processing interrupts
or changing tasks, the Register Pointer speeds
context switching. A special Set Register Pointer
(SRP) instruction sets the contents of the Regis-
ter Pointer.
3.2.1 Error Conditions
Registers must be correctly used because certain
conditions produce inconsistent results and should
be avoided:
REGISTER R253
0
0
0
R6
I
•
Registers
registers.
R243 and R245-R249 are write-only
If an attempt is made to read these
registers, %FF is returned (% is a prefix that
indicates hexadecimal notation).
• When register R253 (Register Pointer) is read,
all as are returned in the least significant
four bits.
-~I
-~I
-~
-~
-~I
I
-~I
-~I
-~l
r-----+-----------------,255
o 0 0 0
253
~--------~----------~
~----------------------~240
1 27
The lower
nibble of
~>-
the register
file address
provided by
the instruction
points to the
specified
register.
1
~------------3
1/0 PORTS
0
Figure 3-3. Working Register Croups
OPC
0
,1
o I INSTRUCTION
(INC
R6)
o I REGISTER ADDRESS (R118)
Figure 3-4. Working Register Addressing
3-2 3047·034, 3047·048

ZiLOG Z8 Specifications

General

Brand	ZiLOG
Model	Z8
Category	Desktop
Language	English