Intel 80286, 80287 manual Bytes and Words in Memory

80286 BASE ARCHITECTURE

Each byte within a word has its own particular address, and the smaller of the two addresses is used as the address of the word. The byte at this lower address contains the eight least significant bits of the word, while the byte at the higher address contains the eight most significant bits. The arrangement of bytes within words is illustrated in figure 2-2.

Note that a word need not be aligned at an even-numbered byte address. This allows maximum flexi- bility in data structures (e.g., records containing mixed byte and word entries) and efficiency in memory utilization. Although actual transfers of data between the processor and memory take place at physi- cally aligned word boundaries, the 80286 converts requests for unaligned words into the appropriate sequences of requests acceptable to the memory interface. Such odd aligned word transfers, however, may impact performance by requiring t'Yo memory cycles to transfer the word rather than one. Data structures (e.g., stacks) should therefore be designed in such a way that word operands are aligned on word boundaries whenever possible for maximum system performance. Due to instruction prefetching and queueing within the CPU, there is no requirement for instructions to be aligned on word bounda- ries and no performance loss if they are not.

Although bytes and words are the fundamental data types of operands, the processor also supports additional interpretations on these bytes or words. Depending on the instruction referencing the operand, the following additional data types can be recognized:

Integer:

A signed binary numeric value contained in an 8-bit byte or a 16-bit word. All operations assume a 2's complement representation. (Signed 32- and 64-bit integers are supported using the 80287 Numeric Data Processor.)

BYTE	MEMORY
ADDRESS'	VALUES
'r		"
E
D
C	FE	WORD AT ADDRESS B
B	06	CONTAINS FE06
		I
A
		BYTE AT ADDRESS 9
9	1F	) CONTAINS 1F
8
7	23	WORD AT ADDRESS 6
6	OB	CONTAINS 230B
		I
5
4
3	74	WORD AT ADDRESS 2
	31
		I
		CONTAINS 74CB
2	CB	IWORD AT ADDRESS 1
		CONTAINS CB31
o		'NOTE:
		ALL VALUES IN HEXADECIMAL

G30108

Figure 2-2.Bytes and Words in Memory

2-3