1.6.4 Basic Instruction Formats
The H8/300H instructions consist of 2-byte (1-word) units. An instruction consists of an operation
field (OP field), a register field (r field), an effective address extension (EA field), and a condition
field (cc).
Operation Field: Indicates the function of the instruction, the effective address, and the operation
to be carried out on the operand. The operation field always includes the first four bits of the
instruction. Some instructions have two operation fields.
Register Field: Specifies a general register. Address registers are specified by 3 bits, data
registers by 3 bits or 4 bits. Some instructions have two register fields. Some have no register
field.
Effective Address Extension: Eight, 16, or 32 bits specifying immediate data, an absolute
address, or a displacement. A 24-bit address or a displacement is treated as 32-bit data in which
the first 8 bits are 0.
Condition Field: Specifies the branching condition of Bcc instructions.
Figure 1-12 shows examples of instruction formats.
Figure 1-12 Instruction Formats
op
op
rn
rm
NOP, RTS, etc.
ADD. Rn, Rm, etc.
MOV @(d:16, Rn), Rm
(1) Operation field only
(2) Operation field and register fields
(3) Operation field, register fields, and effective address extension
rn
rm
op
EA (disp)
(4) Operation field, effective address extension, and condition field
op
cc
EA (disp)
BRA @(d:8, PC)
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