Z86C21 datasheet(13/35 Pages) ZILOG

13 / 35 page

Z86C21 MCU

WITH

8K ROM

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

The upper nibble of the register file address

provided by the register pointer specifies

the active working-register group.

R253

(Register Pointer)

I/O Ports

Specified Working

The lower nibble

of the register

file address

provided by the

instruction points

to the specified

R15 to R0

R15 to R4

R3 to R0

R15 to R0

•

Stack Pointer (Bits 7-0)

R255

Stack Pointer (Bits 15-8)

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

R254

R253

R252

R251

R250

R249

R248

R247

R246

R245

R244

R243

R242

R241

R240

R239

SPL

SPH

FLAGS

IMR

IRQ

IPR

P01M

P3M

P2M

PRE0

PRE1

TMR

SIO

LOCATION

IDENTIFIERS

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.

Figure 12. Register File

Figure 13. Register Pointer

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