Intel 80C186XL, User Manual

Page: 160 / 405

6-5
CHIP-SELECT UNIT
By combining LCS, UCS and MCS3:0, you can cover up to 786 Kbytes of memory address space.
Methods such as those shown in Figure 6-1 on page 6-2 can be used to decode the remaining 256
Kbytes.
The PCS6:0 chip-selects access a contiguous, 896-byte block of memory or I/O address space.
Each chip-select goes active for one-seventh of the block (128 bytes). The start address is pro-
grammed in the PACS register (Figure 6-8 on page 6-10); it can begin on any 1 Kbyte boundary.
A chip-select goes active when it meets all of the following criteria:
1. The chip-select is enabled.
2. The bus cycle status matches the default or programmed type (memory or I/O).
3. The bus cycle address is within the default or programmed block size.
4. The bus cycle is not accessing the Peripheral Control Block.
A memory address applies to memory read, memory write and instruction prefetch bus cycles.
An I/O address applies to I/O read and I/O write bus cycles. Interrupt acknowledge and HALT
bus cycles never activate a chip-select, regardless of the address generated.
After power-on or system reset, only the UCS chip-select is initialized and active (see Figure 6-4).
Figure 6-4. UCS Reset Configuration
1MB
1023K
0
SRDY
NOTE:
1. 3 wait states automatically inserted. Bus READY must be provided.
Processor
Memory
Map
Address
Active For
Top 1 KByte
Data
UCS
UCS
1
ARDY
A1006-0A