stack structure of 8086/88 and advantages,disadvantages

stack structure of 8086 8088 pdf


8086 CPU ARCHITECTURE

The microprocessors functions as the CPU in the stored program model of the digital computer. Its job is to generate all system timing signals and synchronize the transfer of data between memory, I/O, and itself. It accomplishes this task via the three-bus system architecture previously discussed.The microprocessor also has a S/W function. It must recognize, decode, and execute program instructions fetched from the memory unit. This requires an Arithmetic-Logic Unit (ALU) within the CPU to perform arithmetic and logical (AND, OR, NOT, compare, etc) functions.The 8086 CPU is organized as two separate processors, called the Bus Interface Unit (BIU) and the Execution Unit (EU). The BIU provides H/W functions, including generation of the memory and I/O addresses for the transfer of data between the outside world -outside the CPU, that is- and the EU.The EU receives program instruction codes and data from the BIU, executes these instructions, and store the results in the general registers. By passing the data back to the BIU, data can also be stored in a memory location or written to an output device. Note that the EU has no connection to the system buses. It receives and outputs all its data thru the BIU.The execution unit contains the Data and Address registers, the Arithmetic and Logic Unit and theControl Unit. The Bus Interface Unit contains Bus Interface Logic, Segment registers, Memory addressing logicand a Six byte instruction object code queue (4-byte instruction object-code queue in case of 8088microprocessor).The execution unit and the Bus Interface unit operate asynchronously. The EU waits for the instruction object code to be fetched from the memory by the BIU.The BIU fetches or pre-fetches the object code (16-bits at a time) and loads it into the six bytes queue.Whenever the EU is ready to execute a new instruction, it fetches the instruction object code from the front ofthe instruction queue and executes the instruction in specified number of clock periods.If memory or Input/output devices must be accessed in the course of executing an instruction, then the EU informs the BIU of its needs. The BIU completes its operation code (opcode) fetch cycle, if in progress, and executes an appropriate external access machine cycle in response to the EU demand.The BIU is independent of the EU and attempts to keep the six-bytes queue filled with instruction object codes. If two or more of these six bytes are empty, then the BIU executes instruction fetch machine cycles as long as the EU does not have an active request for the bus access pending. If the EU issues a request for the bus access while the BIU is in the middle of an instruction fetch machine cycle, then the BIU will complete the instruction fetch machine cycle before honoring the EU bus access request.The EU does not use machine cycles; it executes instructions in some number of clock periods that are not subjected to any type of machine cycles. The only time clock periods are grouped is clock when the bus control logic wishes to access memory or I/O devices.