General bus operation-microprocessor, Assembly Language

General Bus Operation

The 8086 has a joined data and address bus commonly referred to as a time multiplexed address and data bus. The major reason behind  multiplexing address and data  over the similar pins is the  maximum utilization of processor pins and it facilitates the use of 40 pin standard  DIP package. The bus may be de-multiplexed by using a few transceivers and latches, whenever required. In the following text, a general bus operation cycle is described.

Mainly all the processor bus cycles contain of at least 4 clock cycles. These are referred to as T4, T3, T2 and T1• the address is transmitted by the processor during T1. It is present on the bus just for 1 cycle. During T2, for example- in the next cycle, the bus is tristated for altering the direction of bus for the following data read cycle. The data transfer takes place during T4and T3. In case, an addressed device is slow and shows

'NOT READY' status the wait states Tware inserted between T3 and T4. During wait periodthese  all clock states are known wait states (Tw),idle states (Ti),  or inactive  states. The processor utilize these cycles for internal housekeeping.  The  address  latch  enable  (ALE)  signal  is  emitted  during  T,  by  the bus controller (maximum mode) orthe  processor (minimum mode) depending upon the status of the MN/MX input. The negative edge of this ALE pulse is utilized to separate the address and the status or data information. In maximum mode, the status lines S0, S1 and S2 are utilize to indicate the type of operation as described in the pin description of this unit. Status bits BHE/s7 are multiplexed with BHE and signalhigher order address bits. Address is valid during T1 while the status bits S3  to S7  are valid during T2  through T4. The Figure2.7 shows a general bus operation cycle of 8086.


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