80C186EB/80C188EB Microprocessor User's Manual - CEUNES

OVERVIEW OF THE 80C186 FAMILY ARCHITECTUREThe Execution Unit does not connect directly to the system bus. It obtains instructions from aqueue maintained by the Bus Interface Unit. When an instruction requires access to memory or aperipheral device, the Execution Unit requests the Bus Interface Unit to read and write data. Addressesmanipulated by the Execution Unit are 16 bits wide. The Bus Interface Unit, however,performs an address calculation that allows the Execution Unit to access the full megabyte ofmemory space.To execute an instruction, the Execution Unit must first fetch the object code byte from the instructionqueue and then execute the instruction. If the queue is empty when the Execution Unitis ready to fetch an instruction byte, the Execution Unit waits for the Bus Interface Unit to fetchthe instruction byte.2.1.2 Bus Interface UnitThe 80C186 Modular Core and 80C188 Modular Core Bus Interface Units are functionally identical.They are implemented differently to match the structure and performance characteristics oftheir respective system buses. The Bus Interface Unit executes all external bus cycles. This unitconsists of the segment registers, the Instruction Pointer, the instruction code queue and severalmiscellaneous registers. The Bus Interface Unit transfers data to and from the Execution Unit onthe ALU data bus.The Bus Interface Unit generates a 20-bit physical address in a dedicated adder. The adder shiftsa 16-bit segment value left 4 bits and then adds a 16-bit offset. This offset is derived from combinationsof the pointer registers, the Instruction Pointer and immediate values (see Figure 2-2).Any carry from this addition is ignored.Shift left 4 bits1234Segment Base1 2 3 4 01500202OffsetLogicalAddress190150+0022150= 1192 3 6 20To MemoryPhysical AddressA1500-0AFigure 2-2. Physical Address Generation2-3