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Cadenti
logic family
medium scale integration
metal oxide semiconductor
ac register
accumulator logic
adder circuit
basic computer design
branch unconditionally
flowchart
input and output communication
input output instruction
input register
interrupt cycle
logic adder circuits
logic gates
output register
register and memory
binary code
binary number
clock pulse
data types
decimal numbers
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Input-Output Instructions
Input and output instructions are needed for transferring information to and from AC register, for checking the flag bits, and for controlling the interrupt facility. Input-output instructions have an operation code 1111 and are recognized by the control when D7 = 1 and 7=1. The remaining bits of the instruction specify the particular operation.
The INP instruction transfers the input information from INPR into the eight low-order bits of AC and also clears the input flag to 0. The OUT instruction transfers the eight least significant bits of AC into the output register OUTR and clears the output flag to 0. The next two instructions in Table 5-5 check the status of the flags and cause a skip of the next instruction if the flag is 1. The instruction that is skipped will normally be a branch instruction to return and check the flag again. The branch instruction is not skipped if the flag is 0. If the flag is 1, the branch instruction is skipped and an input or output instruction is executed. The last two instructions set and clear an interrupt enable flip-flop IEN. The purpose of IEN is explained in conjunction with the interrupt operation.