Z86C21 Datasheet, PDF (12/35 Pages) Zilog, Inc. – 8K ROM Z8 CMOS MICROCONTROLLER

FUNCTIONAL DESCRIPTION
Address Space
Z86C21 MCU
WITH 8K ROM
Program Memory. The Z86C21 can address up to 56K
bytes of external program memory (Figure 10). The first 12
bytes of program memory are reserved for the interrupt
vectors. These locations contain six 16-bit vectors that
correspond to the six available interrupts. For ROM mode,
byte 13 to byte 8191 consists of on-chip ROM. At ad-
dresses 8192 and greater, the Z86C21 executes external
program memory fetches. In the ROMless mode, the
Z86C21 can address up to 64K bytes of external program
memory. Program execution begins at external location
000C (HEX) after a reset.
65535
8192
8191
External
ROM and RAM
On-Chip ROM
Data Memory (/DM). The ROM version can address up to
56K bytes of external data memory space beginning at
location 8192. The ROMless version can address up to
64K bytes of external data memory. External data memory
can be included with, or separated from, the external
program memory space. /DM, an optional I/O function that
can be programmed to appear on P34, is used to distin-
guish between data and program memory space (Figure
11). The state of the /DM signal is controlled by the type
instruction being executed. An LDC opcode references
PROGRAM (/DM inactive) memory, and an LDE instruction
references DATA (/DM active Low) memory.
65535
Location of 12
First Byte of
Instruction 11
Executed
After RESET ¹⁰
9
8
7
Interrupt
Vector ₆
(Lower Byte)
5
4
Interrupt
Vector ₃
(Upper Byte)
2
1
0
IRQ5
IRQ5
IRQ4
IRQ4
IRQ3
IRQ3
IRQ2
IRQ2
IRQ1
IRQ1
IRQ0
IRQ0
Figure 10. Program Memory Configuration
External
Data
Memory
8192
8191
0
Not Addressable
Figure 11. Data Memory Configuration
\12