Tandy 1000 HX Technical Reference Manual page 87

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Figure 3. Memory Organization
FUNCTIONAL DESCRIPTION
Memory Organization
The processor provides a 20-bit address to memory
which locates the byte being referenced. The memo-
ry is organized as a linear array of up to 1 million
bytes, addressed as 00000(H) to FFFFF(H). The
memory is logically divided into code, data, extra
data, and stack segments of up to 64K bytes each,
with each segment falling on 16-byte boundaries
(See Figure 3).
All memory references are made relative to base ad-
dresses contained in high speed segment registers.
The segment types were chosen based on the ad-
dressing needs of programs. The segment register
to be selected is automatically chosen according to
the rules of the following table. All information in one
segment type share the same logical attributes (e.g.
code or data). By structuring memory into relocat-
able areas of similar characteristics and by automati-
cally selecting segment registers, programs are
shorter, faster, and more structured.
Word (16-bit) operands can be located on even or
odd address boundaries. For address and data oper-
ands, the least significant byte of the word is stored
in the lower valued address location and the most
significant byte in the next higher address location.
The BIU will automatically execute two fetch or write
cycles for 16-bit operands.
Memory
Reference Used
Instructions
Stack
Local Data
External (Global) Data
Segment
Register Used
CODE (CS)
STACK (SS)
DATA (DS)
EXTRA (ES)
Segment Selection Rule
Automatic with all instruction prefetch.
All stack pushes and pops. Memory references
relative to BP base register except data references.
Data references when: relative to stack, destination
of string operation, or explicity overridden.
Destination of string operations: Explicitly selected
using a segment override.
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