Memory; Microprocessor; Real-Address Mode - IBM 5170 Technical Reference

Memory
The system board has two banks of memory sockets, each
supporting 18 128K by 1 modules for a total maximum memory
size of 512Kb, with parity checking.
Microprocessor
The Intel 80286 Microprocessor has a 24-bit address, 16-bit
memory interface
1 ,
an extensive instruction set, DMA and
interrupt support capabilities, a hardware fixed-point multiply and
divide, integrated memory management, four-level memory
protection, I-gigabyte (1,073,741,824 bytes) of virtual address
space for each task, and two operating modes: the
8086-compatible real-address mode and the protected
virtual-address mode. More detailed descriptions of the
microprocessor may be found in the publications listed in the
Bibliography of this manual.
Real-Address Mode
In the real-address mode, the microprocessor's physical memory
is a contiguous array of up to one megabyte. The microprocessor
addresses memory by generating 20-bit physical addresses.
The selector portion of the pointer is interpreted as the upper 16
bits of a 20-bit segment address. The lower 4 bits of the 20-bit
segment address are always zero. Therefore, segment addresses
begin on multiples of 16 bytes.
All segments in the real-address mode are 64Kb in size and may
be read, written, or executed. An exception or interrupt can
occur
if
data operands or instructions attempt to wrap around the
end of a segment; for example, a word with its low-order byte at
offset FFFF and its high-order byte at 0000. If, in the
real-address mode, the information contained in the segment does
In this manual. the term interface refers to a device that carries signals between
functional units.
1-4
System
Board