Z89135 Datasheet, PDF (30/66 Pages) Zilog, Inc. – Low-Cost DTAD Controller

Z89135/136 (ROMless)
Low-Cost DTAD Controller
FUNCTIONAL DESCRIPTION (Continued)
Register File. The standard Z8 register file consists of four
I/O port registers, 236 general-purpose registers, and 15
control and status registers (R3-R0, R239-R4, and R255-
R241, respectively). The instructions access registers di-
rectly or indirectly through an 8-bit address field. This al-
lows a short, 4-bit register address using the Register
Pointer (Figure 18). In the 4-bit mode, the register file is
divided into 16 working register groups, each occupying 16
continuous locations. The Register Pointer addresses the
starting location of the active working register group (Fig-
ure 19).
Zilog
Note: Register Group E (Registers E0-EF) is only access-
ed through a working register and indirect addressing
modes.
R253 RP
D7 D6 D5 D4 D3 D2 D1 D0
Default setting after RESET = 00000000
Expanded Register Bank
Working Register Group
Figure 18. Register Pointer Register
1-30
r7 r6 r5 r4 r3 r2 r1 r0
Group 15 (F) Control Registers
R255
R253
R240
The upper nibble of the register file address
provided by the register pointer specifies
the active working-register group
Group 14 (E)
Group 13 (D)
R239
R223
Group 4 (4)
Specified Working
Group 3 (3) _{Register Group}
Group 2 (2)
Group 1 (1)
Group 0 (0)
I/O Ports
R79
The upper nibble
of the register
R63 ^{file address}
provided by the
instruction points
R47 ^{to the specified}
working-register
group
R31
R15
R3
R0
Figure 19. Register Pointer
PRELIMINARY
DS97TAD0300