MOTOROLA M68000 FAMILY Programmer's Reference ... - Freescale

SECTION 2
ADDRESSING CAPABILITIES
Most operations take asource operand and destination operand, compute them, and store
the result in the destination location. Single-operand operations take a destination operand,
compute it, and store the result in the destination location. External microprocessor
references to memory are either program references that refer to program space or data
references that refer to data space. They access either instruction words or operands (data
items) for an instruction. Program space is the section of memory that contains the program
instructions and any immediate data operands residing in the instruction stream. Data space
is the section of memory that contains the program data. Data items in the instruction stream
can be accessed with the program counter relative addressing modes; these accesses
classify as program references.
2.1 INSTRUCTION FORMAT
M68000 family instructions consist of at least one word; some have as many as 11 words.
Figure 2-1 illustrates the general composition of an instruction. The first word of the
instruction, called the simple effective address operation word, specifies the length of the
instruction, the effective addressing mode, and the operation to be performed. The
remaining words, called brief and full extension words, further specify the instruction and
operands. These words can be floating-point command words, conditional predicates,
immediate operands, extensions to the effective addressing mode specified in the simple
effective address operation word, branch displacements, bit number or bit field
specifications, special register specifications, trap operands, pack/unpack constants, or
argument counts.
MOTOROLA
15 0
SINGLE EFFECTIVE ADDRESS OPERATION WORD
(ONE WORD, SPECIFIES OPERATION AND MODES)
SPECIAL OPERAND SPECIFIERS
(IF ANY, ONE OR TWO WORDS)
IMMEDIATE OPERAND OR SOURCE EFFECTIVE ADDRESS EXTENSION
(IF ANY, ONE TO SIX WORDS)
DESTINATION EFFECTIVE ADDRESS EXTENSION
(IF ANY, ONE TO SIX WORDS)
Figure 2-1. Instruction Word General Format
M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL
2-1