8x300 design guide - Al Kossow's Bitsavers - Trailing-Edge

OUTPUT OF ALU
o
I I I I I I
v
_L-BITS_
SHIFT LEFT UNTIL BIT 7 IS
IN BIT POSITION 5
L = 4. AS IN PREVIOUS EXAMPLE
MERGED FIELD
'--y-------
'----y-----'
+L-______ ORIGINAL IV BUS DATA
Figure 2-8. Shift and Merge Functions
Because the process is not an end-around-shift, data shifted from
position 0, the MSB, is lost. The number of positions to be shifted is
determined by instruction bits 13-15: the data is left shifted until
the LSB has reached the bit position specified by instruction bits
13-15.
The merge function allows the user to update part of the existing
I/O bus data without affecting the remaining parts of the data byte.
The length of the bit string to be merged with the existing data is
specified by the L operand field, the LSB of the bit string (after
shifting) being specified by instruction bits 13-15.
It must be kept in mind that the I/O bus is "read from" and "written
to" only in 8-bit bytes. The eight bits of information is read from
the I/O bus and latched into the internal I/o latches of the 8X300.
Some number of bits may then be rotated, masked and shifted as
desired. Prior to output to the I/O bus, the desired bits are then
merged with the original information still being retained in the
internal I/O latches. For this reason, a single bit cannot be read
from one device and written to replace just one bit of another
device in one instruction cycle.
2-12