Powerpc Instruction Set And Addressing Modes - Motorola MPC8240 User Manual

Programming Model

5.3.2 PowerPC Instruction Set and Addressing Modes

All PowerPC instructions are encoded as single-word (32-bit) opcodes. Instruction formats
are consistent among all instruction types, permitting efficient decoding to occur in parallel
with operand accesses. This fixed instruction length and consistent format greatly simplifies
instruction pipelining.
5.3.2.1 Calculating Effective Addresses
The effective address (EA) is the 32-bit address computed by the processor when executing
a memory access or branch instruction or when fetching the next sequential instruction.
The PowerPC architecture supports two simple memory addressing modes:
• EA = (rA|0) + offset (including offset = 0) (register indirect with immediate index)
• EA = (rA|0) +7 rB (register indirect with index)
These simple addressing modes allow efficient address generation for memory accesses.
Calculation of the effective address for aligned transfers occurs in a single clock cycle.
For a memory access instruction, if the sum of the effective address and the operand length
exceeds the maximum effective address, the memory operand is considered to wrap around
from the maximum effective address to effective address 0.
Effective address computations for both data and instruction accesses use 32-bit unsigned
binary arithmetic. A carry from bit 0 is ignored in 32-bit implementations.
In addition to the functionality of the MPC603e, the MPC8240 has additional hardware
support for misaligned little-endian accesses. Except for string/multiple load and store
instructions, little-endian load/store accesses not on a word boundary generate exceptions
under the same circumstances as big-endian requests.
5.3.2.2 PowerPC Instruction Set
The PowerPC instructions are divided into the following categories:
• Integer instructions—These include computational and logical instructions.
— Integer arithmetic — divide instructions execute with a shorter latency as
described in Section 5.7, "Instruction Timing."
— Integer compare
— Integer logical
— Integer rotate and shift
• Floating-point instructions—These include floating-point computational
instructions, as well as instructions that affect the FPSCR.
— Floating-point arithmetic
— Floating-point multiply/add
— Floating-point rounding and conversion
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