ROM: (Read only memory) and PROM: (Programmable read only memory)

 ROM: (Read only memory)

• ROM is a read only memory.
• We can not write anything in it.
• In this we can use diode, MOS to store the information.
• Decoder is used to select the address lines
• OE signal is used to enable the output.

PROM: (Programmable read only memory)

• PROM is a programmable ROM
• It is used for permanent storage purpose
• Once we write something in it then we cannot change it. i.e. it become permanent
• We can use diode along with fusible link in series with it to store the information
• We can burns the fuse according to the information stored in it.

Function of signals of 8085.

 Function of signals of 8085.

  (i) HOLD                (ii) HLDA

HOLD and HLDA: HOLD is an active high, input signal used by other controller to request microprocessor about use of address, data and control signals.

The microprocessor in response to HOLD generates a signal to acknowledge the requesting device by HLDA signal. When HLDA is active it indicates that microprocessor has received HOLD request and will relinquish the buses in next clock cycle. The other controller will use buses and upon completion of work will remove HOLD signal, because of this microprocessor will also make HLDA low. The microprocessor takes control of buses half clock cycle after HLDA goes low.

Fetch and execute operation

Fetch and execute operation:
OPCODE fetch cycle: The microprocessor uses this cycle to take the OPCODE of an instruction i.e. fetch OPCODE. In this case, the address of memory location where the OPCODE is stored is given by program counter. This is always the first cycle of any instruction cycle. The OPCODE is taken from memory and transferred to instruction register for decoding and execution.

              The time required to complete this cycle is 4 or 6 T states.

              Operand fetch cycle: In 2 and 3 byte instruction, taking OPCODE, doesn’t complete the instruction fetching; so we require operand fetch cycle. For 2 byte instruction we require two operand fetch cycles. In this cycle the address is given by program counter. But the contents are not transferred to instruction register instead they are stored in temporary registers. The time required to complete the operand fetch cycle is 3 T states.

              Memory read cycle: The microprocessor executes these cycles to read data from memory. The address of memory location is give by instruction. In this case, the program counter is not used, instead, instruction will give address or will specify, where the address is present. IO/M=0.


Memory write cycle: The microprocessor executes these cycles to write data to memory. The address to memory is given by instruction IO/M=0.


I/O read cycle: The microprocessor executes these cycles to read data from I/O device instead of memory. The status of IO/ line is HIGH. The address of port is given by instruction.
I/O write cycle: The microprocessor executes these cycles to write or send data from I/O device. The status of IO/M line is HIGH. The address of port is given by instruction.

Emulator and its uses in the system design.

Emulator is a device computer program, or system that accepts the same inputs and produces the outputs as a given system.
A software application which enables the computer to “emulate’’ a piece of hardware, fooling the computer into thinking that the hardware, such as a CD-ROM, is attached to it. Emulators are often used in situations where there is no CD-ROM present, as in case of some laptops or to test an image of a CD-ROM created by CD burning software such as Nero.

STACK operation in 8085 microprocessor.

STACK operation in 8085 microprocessor.
 
The stack is a reserved area of the memory in RAM where temporary information may be stored. An 8-bit stack pointer is used to hold the address of the most recent stack entry. This location which has the most recent entry is called as the top of the stack.
When the information is written on the stack, the operation is called PUSH. When the information is read from the stack, the operation is called POP. The stack works on the principle of Last in First Out or Fist in Lat Out.

Important features of Motorola 68000 microprocessor.

Important features of Motorola 68000 microprocessor.
Features of Motorola 68000 microprocessors:
The Motorola 68000 is a 32-bit CISC microprocessor.
It includes multiuser microcomputers like the WICAT 150, Tandy TRS – 80 Model 16, and Fortune 32: 16 and single – user workstations.
Multiprocessing control.
Binary coded decimal arithmetic e.g. ABCD and SBCD.
Its software is generally forward with the rest of the line.

Vectored interrupts:

 Vectored interrupts: 
When interrupt request is activated the microprocessor control logic executes ideal machine cycle. During this cycle, it generates starting address of interrupt service routine. These addresses are fixed for different interrupts. This memory address of interrupt service routine is called as vector location and interrupts which use these vector locations are called as vectored interrupts.
                   E.g. TRAP, RST 7.5, RST 6.5, RST 5.5