Real Address Mode; System Timers; Table 6 -1: Channel 0 - Datatech Enterprises Apex 386SX/20 User Manual

Real Address Mode

In real mode the 386SX microprocessor operates as a very fast 8086,
but with a 32-bit extension if desired. Real mode is required primarily
to set up the processor for protected mode operation.
The segmentation unit shifts the selector left four bits and adds the
result to the effective address to form the linear address. This linear
address is limited to 1 megabyte. In addition, real mode has no paging
capability.

System Timers

The system has three programmable timer/counters controlled by the
Intel 8254-2 chip. These are channels 0 through 2 defined as follows:

Table 6 -1: Channel 0

Table 6-2: Channel 1
NOTE: Channel 1 is programmed to generate a 15 microsecond
signal.
Table 6-3: Channel 2
18 Chapter 6: Appendix