Z86E3016PSG Datasheet, PDF (29/348 Pages) Zilog, Inc. – Z86E3016PSG

Z8 CPU
User Manual
11
registers are known as Working Registers. A Register Pointer (one of the
control registers, FDh) contains the base address of the active Working
Register Group. The high nibble of the Register Pointer determines the
current Working Register Group.
When accessing one of the Working Registers, the 4-bit address of the
Working Register is combined within the upper four bits (high nibble) of
the Register Pointer, thus forming the 8-bit actual address. Figure 4 illus-
trates this operation. Because working registers are typically specified by
short format instructions, there are fewer bytes of code required, which
reduces execution time. In addition, when processing interrupts or chang-
ing tasks, the Register Pointer speeds context switching. A special Set
Register Pointer (SRP) instruction sets the contents of the Register
Pointer.
Table 3. Working Register Groups
Register Pointer
(FDh) High Nibble
1111b
1110b
1101b
1100b
1011b
1010b
1001b
1000b
0111b
0110b
0101b
0100b
Working Register Actual Registers
Group (Hex)
(Hex)
F
F0–FF
E
E0–EF
D
D0–DF
C
C0–CF
B
B0–BF
A
A0–AF
9
90–9F
8
80–8F
7
70–7F
6
60–6F
5
50–5F
4
40–4F
UM001602-0904
Address Space