Z8 Microcontrollers
Address Space
ZiLOG
2-4
UM001600-Z8X0599
2.2 Z8 MCU STANDARD REGISTER FILE
(Continued)
Note:
The full register file is shown. Please refer to the selected device product specification for actual file size.
2.2.4 Error Conditions
Registers in the Z8 Standard Register File must be correct-
ly used because certain conditions produce inconsistent
results and should be avoided.
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Registers F3H and F5H-F9H are write-only registers. If
an attempt is made to read these registers, FFH is
returned. Reading any write-only register will return
FFH.
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When register FDH (Register Pointer) is read, the least
significant four bits (lower nibble) will indicate the current
Expanded Register File Bank. (Example: 0000 indicates
the Standard Register File, while 1010 indicates
Expanded Register File Bank A.)
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When Ports 0 and 1 are defined as address outputs,
registers 00H and 01H will return 1s in each address bit
location when read.
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Writing to bits that are defined as timer output, serial
output, or handshake output will have no effect.
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The Z8 instruction DJNZ uses any general-purpose
working register as a counter.
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Logical instructions such as OR and AND require that
the current contents of the operand be read. They
therefore will not function properly on write-only
registers.
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The WDTMR register must be written within the first 60
internal system clocks (SCLK) of operation after a reset.
Figure 2-4. Register Pointer
FF
F0
EF
80
7F
70
6F
60
5F
50
4F
40
3F
30
2F
20
1F
10
0F
R7 R6 R5 R4 R3 R2 R1 R0
Specified Working Register Group
R253
(Register Pointer)
I/O Ports
Working Register Group 1
Working Register Group 0
Working Register Group F
00
The lower nibble
of the register
file address
(provided by the
instruction) points
to the specified
register.
The upper nibble of the register file address,
provided by the register pointer, specifies
the active working-register group.
R15 to R0
R15 to R4
R3 to R0