Organization Of Data In Registers; Data Registers - Motorola MC68030 User Manual

defined by the instruction operation. Coprocessors are designed to support
special computation models that require very specific but widely varying
data operand types and sizes. Hence, coprocessor instructions can specify
operands of any size.
2.2 ORGANIZATION OF DATA IN REGISTERS
The eight data registers can store data operands of 1, 8, 16, 32, and 64 bits,
addresses of 16 or 32 bits, or bit fields of 1 to 32 bits. The seven address
registers and the three stack pointers are used for address operands of 16
or 32 bits. The control registers (SR, VBR, SFC, DFC, CACR, CAAR, CRP, SRP,
TC, TTO, TT1, and MMUSR) vary in size according to function. Coprocessors
may define unique operand sizes and support them with on-chip registers
accordingly.
2.2.1 Data Registers
2-2
Each data register is 32 bits wide. Byte operands occupy the low-order 8 bits,
word operands the low-order 16 bits, and long-word operands the entire 32
bits. When a data register is used as either a source or destination operand,
only the appropriate low-order byte or word (in byte or word operations,
respectively) is used or changed; the remaining high-order portion is neither
used nor changed. The least significant bit of a long-word integer is addressed
as bit zero, and the most significant bit is addressed as bit 31. For bit fields,
the most significant bit is addressed as bit zero, and the least significant bit
is addressed as the width of the field minus one. If the width of the field plus
the offset is greater than 32, the bit field wraps around within the register.
The following illustration shows the organization of various types of data in
the data registers.
Quad-word data consists of two long words: for example, the product of 32-
bit multiply or the quotient of 32-bit divide operations (signed and unsigned).
Quad words may be organized in any two data registers without restrictions
on order or pairing. There are no explicit instructions for the management
of this data type, although the MOVEM instruction can be used to move a
quad word into or out of the registers.
Binary-coded decimal (BCD) data represents decimal numbers in binary form.
Although many BCD codes have been devised, the BCD .instructions of the
M68000 Family support formats in which the four least significant bits consist
of a binary number having the numeric value of the corresponding decimal
number. Two BCD formats are used. In the unpacked BCD format, a byte
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