z89136 ZiLOG Semiconductor, z89136 Datasheet - Page 30

• Manufacturer Part Number: z89136
• Description: Low-cost Dtad Controller
• Manufacturer: ZiLOG Semiconductor

FUNCTIONAL DESCRIPTION (Continued)
Z89135/136 (ROMless)
Low-Cost DTAD Controller
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).
1-30
Group 15 (F) Control Registers
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)
Group 4 (4)
Group 3 (3)
Group 2 (2)
Group 1 (1)
Group 0 (0)
r7 r6 r5 r4
Figure 19. Register Pointer
P R E L I M I N A R Y
I/O Ports
Specified Working
Register Group
r3 r2 r1 r0
Note: Register Group E (Registers E0-EF) is only access-
ed through a working register and indirect addressing
modes.
R253
Default setting after RESET = 00000000
D7 D6 D5 D4 D3 D2 D1 D0
RP
R255
R253
R240
R15
R239
R223
R79
R63
R47
R31
R3
R0
Figure 18. Register Pointer Register
The upper nibble
of the register
file address
provided by the
instruction points
to the specified
working-register
group
Expanded Register Bank
Working Register Group
DS97TAD0300
Zilog