Intel 80C186EA, User Manual

Page: 46 / 417

2-17
OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2.2 SOFTWARE OVERVIEW
All 80C186 Modular Core family members execute the same instructions. This includes all the
8086/8088 instructions plus several additions and enhancements (see Appendix A, “80C186 In-
struction Set Additions and Extensions”). The following sections describe the instructions by cat-
egory and provide a detailed discussion of the operand addressing modes.
Software for 80C186 core family systems need not be written in assembly language. The proces-
sor provides direct hardware support for programs written in the many high-level languages
available. The hardware addressing modes provide straightforward implementations of based
variables, arrays, arrays of structures and other high-level language data constructs. A powerful
set of memory-to-memory string operations allow efficient character data manipulation. Finally,
routines with critical performance requirements can be written in assembly language and linked
with high-level code.
2.2.1 Instruction Set
The 80C186 Modular Core family instructions treat different types of operands uniformly. Nearly
every instruction can operate on either byte or word data. Register, memory and immediate op-
erands can be specified interchangeably in most instructions. Immediate values are exceptions:
they must serve as source operands and not destination operands. Memory variables can be ma-
nipulated (added to, subtracted from, shifted, compared) without being moved into and out of reg-
isters. This saves instructions, registers and execution time in assembly language programs. In
high-level languages, where most variables are memory-based, compilers can produce faster and
shorter object programs.
The 80C186 Modular Core family instruction set can be viewed as existing on two levels. One is
the assembly level and the other is the machine level. To the assembly language programmer, the
80C186 Modular Core family appears to have about 100 instructions. One MOV (data move) in-
struction, for example, transfers a byte or a word from a register, a memory location or an imme-
diate value to either a register or a memory location. The 80C186 Modular Core family CPUs,
however, recognize 28 different machine versions of the MOV instruction.
The two levels of instruction sets address two requirements: efficiency and simplicity. Approxi-
mately 300 forms of machine-level instructions make very efficient use of storage. For example,
the machine instruction that increments a memory operand is three or four bytes long because the
address of the operand must be encoded in the instruction. Incrementing a register, however, re-
quires less information, so the instruction can be shorter. The 80C186 Core family has eight sin-
gle-byte machine-level instructions that increment different 16-bit registers.
The assembly level instructions simplify the programmer’s view of the instruction set. The pro-
grammer writes one form of an INC (increment) instruction and the assembler examines the op-
erand to determine which machine level instruction to generate. The following paragraphs
provide a functional description of the assembly-level instructions.