4/17/98 6-9 Addressing Modes and Data Types
6.4 Data Types in XA
The XA uses the following types of data:
• Bits
• 4/5-bit signed integers
• 8-bit (byte) signed and unsigned integers
• 8-bit, two digit BCD numbers
• 16-bit (word) signed and unsigned integers
• 10-bit address for bit-addressing in data memory and SFR space
• 24-bit effective address comprising of 16-bit address and 8-bit segment select. See addressing
modes for more information.
A byte consists of 8-bits. A word is a 16-bit value consisting of two contiguous bytes. A double
word consists of two 16-bit words packed in two contiguous words in memory.
Negative integers are represented in twos complement form. 4-bit signed integers (sign extended
to byte/word) are used as immediate operands in MOVS and ADDS instructions.
Binary coded decimal numbers are packed, 2 digits per byte. BCD operations use byte operands.
6.5 Instruction Set Overview
The XA uses a powerful and efficient instruction set, offering several different types of
addressing modes. A versatile set of “branch” and “jump” instructions are available for
controlling program flow based on register or memory contents. Special emphasis has been
placed on the instruction support of structured high-level languages and real-time multi-tasking
operating systems.
This section discusses the set of instructions provided in the XA microcontroller, and also shows
how to use them. It includes descriptions of the instruction format and the operands used by the
instructions. After a summary of the instructions by category, the section provides a detailed
description of the operation of each instruction, in alphabetical order.
Five summary tables are provided that describes the available instructions. The first table is a
summary of instructions available in the XA along with their common usage. The second and
third table are tables of addressing modes and operands, and the instruction type they pertain to.
A fourth table that lists the summary of status flags update by different instructions. A fifth table
lists the available instructions with their different addressing modes and briefly describes what
each instruction does along with the number of bytes, and number of clocks required for each
instruction.
The formats have been chosen to optimize the length and execution speed of those instructions
that would be used the most often in critical code. Only the first and sometimes the second byte
of an instruction are used for operation encoding. The length of the instruction and the first
execution cycle activity are determined from the first byte. Instruction bytes following the first
two bytes (if any) are always immediate operands, such as addresses, relative displacements,
offsets, bit addresses, and immediate data.
To Next Page
Loading...