Some signals to be remembered:
Consider we have an address to be processed. But how do the processors know whether the address is for memory or I/O functions? For this purpose a status signal called IO/M’ is used. This distinguishes whether the address is for memory or IO. When this pin goes high, the address is for an I/O device. While the pin goes low, the address is assigned for the memory.
This is an active low signal. That is, an operation is performed when the signal goes low. This signal is used to control READ operation of the microprocessor. When this pin goes low the microprocessor reads the data from memory or I/O device.
WR’ is also an active low signal which controls the write operations of the microprocessor. When this pin goes low, the data is written to the memory or I/O device.
READY is used by the microprocessor to check whether a peripheral is ready to accept or transfer data. A peripheral may be a LCD display or analog to digital converter or any other. These peripherals are connected to microprocessor using the READY pin. If READY is high then the periphery is ready for data transfer. If not the microprocessor waits until READY goes high.
This indicates if any other device is requesting the use of address and data bus. Consider two peripheral devices. One is the LCD and the other Analog to Digital converter. Suppose if analog to digital converter is using the address and data bus and if LCD requests the use of address and data bus by giving HOLD signal, then the microprocessor transfers the control to the LCD as soon as the current cycle is over. After the LCD process is over, the control is transferred back to analog and digital converter.
HLDA is the acknowledgment signal for HOLD. It indicates whether the HOLD signal is received or not. After the execution of HOLD request, HLDA goes low.
INTR is an interrupt request signal. It has the lowest priority among the interrupts. INTR can be enabled or disabled by using software. Whenever INTR goes high the microprocessor completes the current instruction which is being executed and then acknowledges the INTR signal and processes it.
Whenever the microprocessor receives interrupt signal. It has to be acknowledged. This acknowledgement is done by INTA’. So whenever the interrupt is received INTA’ goes high.