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

Data Organization and Addressing Capabilities

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, TT0, 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

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 managment 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 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 contains one digit; the four least significant bits contain the binary value
and the four most significant bits are undefined. Each byte of the packed BCD format
contains two digits; the least significant four bits contain the least significant digit.
2-2 MC68030 USER'S MANUAL MOTOROLA