Bit Field Operations - Motorola MC68030 User Manual

3.5.3
Bit Field Operations
One data type provided by the MC68030 is the bit field, consisting of as many
as 32 consecutive bits. A bit field is defined by an offset from an effective
address and a width value. The offset is a value in the range of - 2
31
through
2
31
-1 from the most significant bit (bit 7) at the effective address. The width
is a positive number, 1-32. The most significant bit of a bit field is bit 0; the
bits number in a direction opposite to the bits of an integer.
The instruction set includes eight instructions that have bit field operands.
The insert bit field (BFINS) instruction inserts a bit field stored in a register
into a bit field. The extract bit field signed (BFEXTS) instruction loads a bit
field into the least significant bits of a register and extends the sign to the
left, filling the register. The extract bit field unsigned (BFEXTU) also loads a
bit field, but zero fills the unused portion of the destination register.
The set bit field (BFSET) instruction sets all the bits of a field to ones. The
clear bit field (BFCLR) instruction clears a field. The change bit field (BFCHG)
instruction complements all the bits in a bit field. These three instructions
all test the previous value of the bit field, setting the condition codes ac-
cordingly. The test bit field (BFTST) instruction tests the value in the field,
setting the condition codes appropriately without altering the bit field. The
find first one in bit field (BFFFO) instruction scans a bit field from bit 0 to the
right until it finds a bit set to one and loads the bit offset of the first set bit
into the specified data register. If no bits in the field are set, the field offset
and the field width is loaded into the register.
An important application of bit field instructions is the manipulation of the
exponent field in a floating-point number. In the IEEE standard format, the
most significant bit is the sign bit of the mantissa. The exponent value begins
at the next most significant bit position; the exponent field does not begin
on a byte boundary. The extract bit field (BFEXTU) instruction and the BFTST
instruction are the most useful for this application, but other bit field instruc-
tions can also be used.
Programming of input and output operations to peripherals requires testing,
setting, and inserting of bit fields in the control registers of the peripherals,
which is another application for bit field instructions. However, control reg-
ister locations are not memory locations; therefore, it is not always possible
to insert or extract bit fields of a register without affecting other fields within
the register.
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MC68030 USER'S MANUAL
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