FMS7401 Datasheet, PDF (51/80 Pages) Fairchild Semiconductor – Digital Power Controller

PRODUCT SPECIFICATION
FMS7401/7401L
10.2 Addressing Modes
The microcontroller core has seven instruction addressing modes: inherent, immediate, direct, indirect, indexed, absolute jump
and relative jump (see Table 24).
Inherent
The inherent addressing mode instructions either have no operand associated or the contents of the operand are already known
to the microcontroller core. The microcontroller core then inherently knows how to execute the instruction without needing
any additional information provided by additional operands.
Immediate
The immediate addressing mode instructions contain a 3-bit,² 8-bit or 12-bit³ immediate field as an operand. Immediate
addressing is so-named because the value needed to complete the instruction is provided immediately to the core within the
instruction code. That is to say, the instruction itself dictates what the data value is to be e.g. stored in a register.
Direct
The direct addressing mode instructions contain an 8-bit address operand that directly points to a location within the data
memory space. Direct addressing is so-named because the value needed to complete the instruction must be directly accessed
by the core from the memory address provided by the instruction code.
Indirect
The indirect addressing mode instructions use the content in XLO, X[7:0], to address a specific location within the data mem-
ory space (0x00 – 0xFF).⁴ Indirect addressing is so-named because the value needed to complete the instruction must be
retrieved indirectly by the core from the address provided by the X-pointer.
Indexed
The indexed offset addressing mode instructions add an 8-bit unsigned offset value to the X-pointer yielding a new effective
address to select a specific location anywhere within the memory map (both program and data memory space, 0x000-0xFFF).
Indexed addressing expands the functions of indirect addressing by providing the only means to access the data stored within
the program memory space.
Absolute
The absolute jump addressing mode instructions (e.g. JMP and JSR) replace the program counter with the value in the operand
field. This allows jumping to any location within the program memory space.⁵
Relative
The opcode instruction field for the relative jump addressing mode instruction, JP, is calculated from the distance to the abso-
lute program memory location in the operand addressing the next instruction to be executed. The base opcode for JP is 0xC0
where bit 5 indicates the direction within memory to jump. Bits 4 to 0 indicate the number of bytes to jump where the maxi-
mum distance is 31 bytes. If bit 5 is zero, the address for the next instruction executed is determined by subtracting the lower 5
bits of the opcode (0xC1-0xDF) from the program counter; otherwise, the lower 5 bits of the opcode (0xE0-0xFF) are added to
the program counter.⁶
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