Z8 Microcontrollers
Address Space
ZiLOG
2-6
UM001600-Z8X0599
2.3 Z8 EXPANDED REGISTER FILE
(Continued)
Currently, three out of the possible sixteen Z8 ERF Banks
have been implemented. ERF Bank 0, also known as the
Z8 Standard Register File, has all 256 bytes defined (Fig-
ure 2-1). Only Working Register Group 0 (register address-
es 00H to 0FH) have been defined for ERF Bank C and
ERF Bank F (Table 2-4). All other working register groups
in ERF Banks C and F, as well as the remaining thirteen
ERF Banks, are not implemented. All are reserved for fu-
ture use.
When an ERF Bank is selected, register addresses 00H to
0FH access those sixteen ERF Bank registers – in effect
replacing the first sixteen locations of the Z8 Standard
Register File.
For example, if ERF Bank C is selected, the Z8 Standard
Registers 00H through 0FH are no longer accessible. Reg-
isters 00H through 0FH are now the 16 registers from ERF
Bank C, Working Register Group 0. No other Z8 Standard
Registers are effected since only Working Register Group
0 is implemented in ERF Bank C.
Access to the ERF is accomplished through the Register
Pointer (FDH). The lower nibble of the Register Pointer de-
termines the ERF Bank while the upper nibble determines
the Working Register Group within the register file (Figure
2-6).
The value of the lower nibble in the Register Pointer (FDH)
corresponds to the ERF Bank identification. Table 2.3
shows the lower nibble value and the register file assigned
to it.
Figure 2-6. Register Pointer (FDH) Example
0 1 1 1
1 1 0 0
Working
Register
Group
Select ERF Bank C(H)
Working Register Group 7(H)
Expanded
Register
Bank
Table 2-3. ERF Bank Address
Register Pointer
(FDH)
Low Nibble
0000(B)
0001(B)
0010(B)
0011(B)
0100(B)
0101(B)
0110(B)
0111(B)
1000(B)
1001(B)
1010(B)
1011(B)
1100(B)
1101(B)
1110(B)
1111(B)
Hex
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
Register File
Z8 Standard Register File *
Expanded Register File Bank 1
Expanded Register File Bank 2
Expanded Register File Bank 3
Expanded Register File Bank 4
Expanded Register File Bank 5
Expanded Register File Bank 6
Expanded Register File Bank 7
Expanded Register File Bank 8
Expanded Register File Bank 9
Expanded Register File Bank A
Expanded Register File Bank B
Expanded Register File Bank C
Expanded Register File Bank D
Expanded Register File Bank E
Expanded Register File Bank F
Note:
The Z8 Standard Register File is equivalent to Expanded
Register File Bank 0.