INTERNAL ARCHITECTURE
The Execution Unit in turn consists of three subunits:
•Control Unit
•Data Unit
•Protection Test Unit
Figure 2-2 shows the organization of these units. This chapter describes the function of each unit, as well as interactions between units.
2.1 BUS INTERFACE UNIT
The Bus Interface Unit provides the interface between the Inte1386 DX microprocessor and its environment. It accepts internal requests for code fetches (from the Code Prefetch Unit) and data transfers (from the Execution Unit), and prioritizes the requests. At the same time, it generates or processes the signals to perform the current bus cycle. These signals include the address, data, and control outputs for accessing external memory and I/O. The Bus Interface Unit also controls the interface to external bus masters and coprocessors.
2.2 CODE PREFETCH UNIT
The Code Prefetch Unit performs the program look ahead function of the Inte1386 DX microprocessor. When the Bus Interface Unit is not performing bus cycles to execute an instruction, the Code Prefetch Unit uses the Bus Interface Unit to fetch sequentially along the instruction byte stream. These prefetched instructions are stored in the
Code prefetches are given a lower priority than data transfers; assuming zero wait state memory access, prefetch activity never delays execution. On the other hand, if there is no data transfer requested, prefetching uses bus cycles that would otherwise be idle. Instruction prefetching reduces to practically zero the time that the processor spends waiting for the next instruction.
2.3 INSTRUCTION DECODE UNIT
The Instruction Decode Unit takes instruction stream bytes from the Prefetch Queue and translates them into microcode. The decoded instructions are then stored in a three- deep Instruction Queue (FIFO) to await processing by the Execution Unit. Immediate data and opcode offsets are also taken from the Prefetch Queue. The decode unit works in parallel with the other units and begins decoding when there is a free slot in the FIFO and there are bytes in the prefetch queue. Opcodes can be decoded at a rate of one byte per clock. Immediate data and offsets· can be decoded in one clock regardless of their length.
