The remaining 16 bits are used for highly encoded micro-operations. The
format is much like a machine instruction, with a variable-length opcode and one or
more operands.
IBM 3033 Microinstruction Execution
The standard IBM 3033 control memory consists of 4K words.The first half of these
(0000–07FF) contain 108-bit microinstructions, while the remainder (0800–0FFF)
are used to store 126-bit microinstructions. The format is depicted in Figure 16.16.
Although this is a rather horizontal format, encoding is still extensively used. The
key fields of that format are summarized in Table 16.6.
The ALU operates on inputs from four dedicated, non-user-visible registers,A,
B, C, and D. The microinstruction format contains fields for loading these registers
from user-visible registers, performing an ALU function, and specifying a user-visible
register for storing the result. There are also fields for loading and storing data between
registers and memory.
The sequencing mechanism for the IBM 3033 was discussed in Section 16.2.