Chapter 3 Terminology

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Chapter 3 Terminology

ismithers
Hi, I'm working my way through chapter 3 and I wanted to ask about the terminology of bit and Bit which appear to be used to reference different things. A bit with a lowercase 'b' refers to the definition I'm accustomed to, which is an atomic value of information in computing. But then later there are figures in the book, such as 3.1 and 3.2 that have items labelled as Bit with an uppercase 'B'. These seem to refer to a part of a chip - is this just a lexical form in the book, or are there bits and Bits and they are different?

Also I'm confused about figure 3.1 and the text in the paragraph below that references it. It says that the right-most image is the correct implementation as it uses a Mux chip. However it says that if we wish to start storing a new value, we can put the value into the in pin, and feed the selector pin with a value of 1. On page 21 the Mux diagram (and in fact my implementation) says that the the selector pin with a value of 1, will feed the input from pin b through to the out pin. Why in 3.1 does it feed what looks like pin a through to the out pin?

The only thing I can think of is that 3.1 has a Mux chip but show from a different perspective - ie the underside. I don't really think that is the answer, but it's all I can think of because on page 21 the selector pin is shown on the bottom of the diagram, but in 3.1 its on the top. That would effectively reverse the a and b pin positions. Small thing, but I was just trying to be thorough and make sure I'm following what I've learned in the prior chapters.

Thanks!
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Re: Chapter 3 Terminology

cadet1620
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Lowercase 'bit' is the generic bit you know. Uppercase 'Bit' refers to a chip that you will build, Bit.hdl. Think of Bit as a 1-bit memory cell; you'll use 16 of them to make Register.hdl.

Figure 3.1 is confusing, as you point out. And you have solved the conundrum correctly; the mux is flipped from how it's been drawn earlier.

Schematics usually label at least one of the mux inputs (or demux outputs) so that there is no ambiguity. The select input can be top or bottom, whichever is more convenient for a particular schematic. You will also see muxes and demuxes drawn as trapezoids instead of triangles.

--Mark
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