Processor-memory: Data may be transferred from processor to memory or from memory to processor. Processor-I/O: Data may be transferred to or from a peripheral device by transferring between the processor and an I/O module. Data processing: The processor may perform some arithmetic or logic operation on data. Control: An instruction may specify that the sequence of execution be altered.
Instruction address calculation (iac): Determine the address of the next instruction to be executed. Instruction fetch (if): Read instruction from its memory location into the processor. Instruction operation decoding (iod): Analyze instruction to determine type of operation to be performed and operand(s) to be used. Operand address calculation (oac): If the operation involves reference to an operand in memory or available via I/O, then determine the address of the operand. Operand fetch (of): Fetch the operand from memory or read it in from I/O. Data operation (do): Perform the operation indicated in the instruction. Operand store (os): Write the result into memory or out to I/O.
Disable all interrupts while an interrupt is being processed. (2) Define priorities for interrupts and to allow an interrupt of higher priority to cause a lower-priority interrupt handler to be itself interrupted.
Memory to processor: The processor reads an instruction or a unit of data from memory. Processor to memory: The processor writes a unit of data to memory. I/O to processor: The processor reads data from an I/O device via an I/O module. Processor to I/O: The processor sends data to the I/O device. I/O to or from memory: For these two cases, an I/O module is allowed to exchange data directly with memory, without going through the processor, using direct memory access (DMA).
With multiple buses, there are fewer devices per bus. This (1) reduces propagation delay, because each bus can be shorter, and (2) reduces bottleneck effects.
a.224 = 16 MBytes
b.(1)If the local address bus is 32 bits, the whole address can be transferred at once and decoded in memory. However, since the data bus is only 16 bits, it will require 2 cycles to fetch a 32-bit instruction or operand. (2)The 16 bits of the address placed on the address bus can’t access the whole memory. Thus a more complex memory interface control is needed to latch the first part of the address and then the second part (since the microprocessor will end in two steps). For a 32-bit address, one may assume the first half will decode to access a “row” in memory, while the second half is sent later to access a “column” in memory. In addition to the two-step address operation, the microprocessor will need 2 cycles to fetch the 32 bit instruction/operand. c.The program counter must be at least 24 bits. Typically, a 32-bit microprocessor will have a 32-bit external address bus and a 32-bit program counter, unless on-chip segment registers are used that may work with a smaller program counter. If the instruction register is to contain the whole instruction, it will have to be 32-bits long; if it will contain only the op code (called the op code register) then it will have to be 8 bits long.
3.4In cases (a) and (b), the microprocessor will be able to access 216 = 64K bytes; the only difference is that with an 8-bit memory each access will transfer a byte, while with a 16-bit memory an access may transfer a byte or a 16-byte word. For case (c), separate input and output instructions are needed, whose execution will generate separate “I/O signals” (different from the “memory signals” generated with the execution of memory-type instructions); at a minimum, one additional output pin will be required to carry this new signal. For case (d), it can support 28 = 256 input and 28 = 256 output byte ports and the same number of input and output 16-bit ports; in either case, the distinction between an input and an output port is defined by the different signal that the executed input or output instruction generated.