Z86C21 Datasheet, PDF (13/35 Pages) Zilog, Inc. – 8K ROM Z8 CMOS MICROCONTROLLER

Register File. The Register File consists of four I/O port
registers, 236 general-purpose registers and 16 control
and status registers (Figure 12). The instructions can
access registers directly or indirectly through an 8-bit
address field. The Z86C21 also allows short 4-bit register
addressing using the Register Pointer (Figure 13). In the
4-bit mode, the Register File is divided into 16 working
Z86C21 MCU
WITH 8K ROM
register groups, each occupying 16 continuous locations.
The Register Pointer addresses the starting location of the
active working-register group. For the reset and power-up
conditions of the Register File, see Figure 14.
Note: Register Bank E0-EF can only be accessed through
working registers and indirect addressing modes.
LOCATION
R255
R254
R253
R252
R251
R250
R249
R248
R247
R246
R245
R244
R243
R242
R241
R240
R239
R4
R3
R2
R1
R0
Stack Pointer (Bits 7-0)
Stack Pointer (Bits 15-8)
Register Pointer
Program Control Flags
Interrupt Mask Register
Interrupt Request Register
Interrupt Priority Register
Ports 0-1 Mode
Port 3 Mode
Port 2 Mode
T0 Prescaler
Timer/Counter0
T1 Prescaler
Timer/Counter1
Timer Mode
Serial I/O
General-Purpose
Registers
Port 3
Port 2
Port 1
Port 0
IDENTIFIERS
SPL
SPH
RP
FLAGS
IMR
IRQ
IPR
P01M
P3M
P2M
PRE0
T0
PRE1
T1
TMR
SIO
P3
P2
P1
P0
r7 r6 r5 r4
r3 r2 r1 r0
R253
(Register Pointer)
The upper nibble of the register file address
provided by the register pointer specifies
the active working-register group.
FF
Register Group F
F0
•
•
•
•
•
•
•
•
R15 to R0
•
•
•
•
•
Specified Working
•
Register Group
2F
20
1F
Register Group 1
10
0F
Register Group 0
I/O Ports
00
The lower nibble
of the register
file address
provided by the
instruction points
to the specified
register.
R15 to R0
R15 to R4
R3 to R0
Figure 13. Register Pointer
Figure 12. Register File
13