LSI-11 CONTROL UNIT ORGANIZATION The LSI-11 is the first member of the
PDP-11 family that was offered as a single-board processor. The board contains
three LSI chips, an internal bus known as the microinstruction bus (MIB), and some
additional interfacing logic.
Figure 16.13 depicts, in simplified form, the organization of the LSI-11
processor. The three chips are the data, control, and control store chips. The data
chip contains an 8-bitALU, twenty-six 8-bit registers, and storage for several condition
codes. Sixteen of the registers are used to implement the eight 16-bit
general-purpose registers of the PDP-11. Others include a program status word,
memory address register (MAR), and memory buffer register. Because the ALU
deals with only 8 bits at a time, two passes through the ALU are required
to implement a 16-bit PDP-11 arithmetic operation. This is controlled by the
microprogram.
The control store chip or chips contain the 22-bit-wide control memory. The
control chip contains the logic for sequencing and executing microinstructions. It
contains the control address register,