AMD Am186TMER, Am188TMER user manual Segments, Data Types, Segment Register Selection Rules, Used

2.5SEGMENTS

The Am186ER and Am188ER microcontrollers use four segment registers:

1.Data Segment (DS): The processor assumes that all accesses to the program’s variables are from the 64K space pointed to by the DS Register. The data segment holds data, operands, etc.

2.Code Segment (CS): This 64K space is the default location for all instructions. All code must be executed from the code segment.

3.Stack Segment (SS): The processor uses the SS Register to perform operations that involve the stack, such as pushes and pops. The stack segment is used for temporary space.

4.Extra Segment (ES): Usually this segment is used for large string operations and for large data structures. Certain string instructions assume the extra segment as the segment portion of the address. The extra segment is also used (by using segment override) as a spare data segment.

When a segment is not defined for a data movement instruction, it’s assumed to be a data segment. An instruction prefix can be used to override the segment register. For speed and compact instruction encoding, the segment register used for physical address generation is implied by the addressing mode used (see Table 2-2).

Table 2-2	Segment Register Selection Rules
	Memory Reference	Segment Register	Implicit Segment Selection Rule
	Needed	Used
	Local Data	Data (DS)	All data references
	Instructions	Code (CS)	Instructions (including immediate data)
	Stack	Stack (SS)	All stack pushes and pops
			Any memory references that use the BP Register
	External Data (Global)	Extra (ES)	All string instruction references that use the DI Regis-
			ter as an index

2.6DATA TYPES

The Am186ER and Am188ER microcontrollers directly support the following data types:

νInteger—A signed binary numeric value contained in an 8-bit byte or a 16-bit word. All operations assume a two’s complement representation.

νOrdinal—An unsigned binary numeric value contained in an 8-bit byte or a 16-bit word.

νDouble Word—A signed binary numeric value contained in two sequential 16-bit addresses, or in a DX::AX register pair.

νQuad Word—A signed binary numeric value contained in four sequential 16-bit addresses.

νBinary-Coded Decimal (BCD)—An unpacked byte representation of the decimal digits 0–9.

νASCII—A byte representation of alphanumeric and control characters using the ASCII standard of character representation.

νPacked BCD—A packed byte representation of two decimal digits (0–9). One digit is stored in each nibble (4 bits) of the byte.

2-8 Programming