
MOTOROLA
Chapter 2. Programming Model
2-13
infinities, denormalized numbers, and NaNs follow conventions described in the following
sections.
Although the double-precision format specifies an 11-bit exponent, exponent arithmetic
uses two additional bit positions to avoid potential transient overflow conditions. An extra
bit is required when denormalized double-precision numbers are prenormalized. A second
bit is required to permit computation of the adjusted exponent value in the following
examples when the corresponding exception enable bit is 1:
Underflow during multiplication using a denormalized factor
Overflow during division using a denormalized divisor
2.2.2 Data Organization in Memory and Data Transfers
Bytes in memory are numbered consecutively starting with 0. Each number is the address
of the corresponding byte.
Memory operands may be bytes, half words, words, or double words, or, for the load/store
multiple and move assist instructions, a sequence of bytes or words. The address of a
memory operand is the address of its first byte (that is, of its lowest-numbered byte).
Operand length is implicit for each instruction.
2.2.3 Alignment and Misaligned Accesses
The operand of a single-register memory access instruction has a natural alignment
boundary equal to the operand length. In other words, the “natural” address of an operand
is an integral multiple of the operand length. A memory operand is said to be aligned if it
is aligned at its natural boundary; otherwise it is misaligned.
Operands for single-register memory access instructions have the characteristics shown in
Table 2-8. (Although not permitted as memory operands, quad words are shown because
quad-word alignment is desirable for certain memory operands.)
Table 2-8. Memory Operands
Operand
Length
Addr[28–31]
If Aligned
Byte
8 bits
xxxx
Half word
2 bytes
xxx0
Word
4 bytes
xx00
Double word
8 bytes
x000
Quad word
16 bytes
0000
Note
: An “x” in an address bit position indicates that the bit can
be 0 or 1 independent of the state of other bits in the
address.