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MC68349 USER’S MANUAL
MOTOROLA
instruction pipeline when fetching from a 32-bit port. When fetching from a 16-bit port, this
additional word is disabled. The performance of the CPU32+ on a 16-bit bus is the same
as the CPU32 performance.
The CPU32+ supports byte-misaligned operands. Since operands can reside at any byte
boundary, they may occasionally become misaligned. A byte operand is properly aligned
at any address; a word operand is misaligned at a odd address; a long-word operand is
misaligned at an address that is not evenly divisible by four. Devices such as the
MC68302, MC68000/8, MC68010, and CPU32-based M68300 family processors allow
long-word operand transfers at odd-word addresses, but force exceptions if word or long-
word operand transfers are attempted at odd-byte addresses. Although the CPU32+ does
not enforce any alignment restrictions for data operands (including program counter (PC)
relative data addresses), some performance degradation occurs when additional bus
cycles are required for long-word or word operands that are misaligned. For maximum
performance, data items should be aligned on their natural boundaries. All instruction
words and extension words must reside on word boundaries. Attempting to prefetch an
instruction word at an odd address causes an address error exception.
The CPU32+ has four bits (SIZ1,SIZ0 and SZC1,SZC0) in the software status word
(SSW) that are new or have changed definitions due to the 32-bit bus width and
elimination of alignment restrictions.
The CPU32+ offers low power consumption. The CPU32+ is implemented in high-density
complementary metal-oxide semiconductor (HCMOS) technology, providing low power
use during normal operation. During periods of inactivity, the low-power stop (LPSTOP)
instruction can be executed, shutting down the CPU32+ and other IMB modules, greatly
reducing power consumption.
Ease of programming is an important consideration when using an integrated processor.
The CPU32+ instruction format reflects a predominant register-memory interaction
philosophy. All data resources are available to operations that require them. The
programming model includes eight multifunction data registers and seven general-purpose
addressing registers. The data registers support 8-bit (byte), 16-bit (word), and 32-bit
(long-word) operand lengths for all operations. Address manipulation is supported by word
and long-word operations. Although the PC and stack pointers (SP) are special-purpose
registers, they are also available for most data addressing activities. Ease of program
checking and diagnosis is enhanced by trace and trap capabilities at the instruction level.
As processor applications become more complex and programs become larger, high-level
languages (HLLs) become the system designer's choice in programming languages. HLLs
aid in the rapid development of complex algorithms with less error and are readily
portable. The CPU32+ instruction set efficiently supports HLLs.