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8251 Programmable/Communication Interface
As an instance of a serial interface device let us suppose Intel's 8251 A programmable communication interfaces. The 8251A is diagrammed in given figure. It is capable of being programmed for synchronous / asynchronous communication. The data that is in buffer and data-out buffer registers share the similar port address. For input, the serial bit stream is arriving on the R x D pin is shifted into the receiver shift register and then the data bits are transferred to the data-in buffer register, where they might be input by the CPU. On the other hand on output the data bits put in the data-out buffer register by the CPU are transferred to the transmitter shift register and, along with the essential synchronization bits, are shifted out through the T x D pin. Among other things the contents of the mode register, which are begin by the executing program, determine whether the 8251A is in synchronous mode or asynchronous mode and the format of the characters being received and transmitted. The control register, which is set by the program, controls the operation of the interface, and the status register makes sure information available to the executing program. Obviously, the sync character registers are for storing the sync characters required for synchronous communication.
Even though all 7 of the registers on the left side of Figure 4.7 may be accessed by the processor, the 8251A is associated with only 2 port addresses. The C/D pin is linked to the address line AO and AO differentiates the 2 port addresses. The 8251A internally interprets the C/D, RD, and WR signals as follows:
Assume that the registers are initialized to EAX=12345h,EBX =9528h ECX=1275h,EDX=3001h sub AH,AH sub DH,DH mov DL,AL mov CL,3 shl DX,CL shl AX,1 add DX,AX
programs
2. Write a program to separate out positive and negative numbers from a given series of 16-bit hexadecimal numbers.
8279 Keyword /Display Controller : Figure shows the structure of 8279 and its interface to the bus. Addressing is according to the table given below. CS RD
to separate positive and negative numbers
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