The daisy chain order established the priority of the device. Generally the first logic gate is for the highest priority.
#Pdp 11 emulator mac series
A daisy chain is a sequence of logic gates arranged in series to prioritize events. A device would request an interrupt by asserting an input into an interrupt daisy chain. The interrupt system was intentionally designed to be as simple as possible, while assuring that no event in an interrupt sequence could be missed. All input and output devices were mapped to addresses in memory, so in addition, no special I/O instructions were needed. In the most radical departure from other, earlier computers, the PDP-11 had no dedicated bus for input/output it had only a memory bus. Any register could be used for most purposes, although register 6 was elected as the stack pointer for subroutines, and register 7 was used as program counter.
There were 8 general-purpose registers numbered 0 through 7. All op-codes operated with any combination of address modes. Each is instruction separated into two six-bit address modes and a four-bit op-code. In some logical sense, the set of addressing modes provided one "basis" and the set of operations provided another. He did not have to learn a list of exceptions or special cases in which an operation had a special or restricted set of addressing modes. He could then predict that any access method (or, "addressing mode") would work with any operation. This made the computer much easier to program because a programmer could memorize the operations, and the methods of accessing operands. This property is called "orthogonality" in an allusion to linear mathematics. addition, subtraction, etc., was cleanly and precisely separated from the method to locate the operands. First, the instruction set was one of the most orthogonal ever designed.
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