Elements of Computing Systems Chapter 3

Preamble

• Welcome Sam
• Quick overview of the chapter
• DFF: primitive and possible implementations
• How do computer clocks actually work?
  • Capacitors and filters (Chris L)
  • Crystal with resonant frequency (James M)
• A cycle is from the beginning of a tick to the end of the tock
• How does the simulator’s clock work (Appendix A.7)
  • During a tick the inputs are read, on tock the outputs are set
• Tuning a clock cycle such that all part of the circuit can be reached
• Chips produce junk between ticks
• A DFF is effectively like a delay line
• The “invalid design” with fan-in 2 in Figure 3.1 is really a short circuit
  • The Mux both isolates the circuit and allows our behaviour
• Does a Latch relate to flip-flops?
  • Wikipedia page is fairly revealing
• Addressing is introduced without too much foreshadowing of how it’ll be used later on
• Multiplexers are about where something goes our load pins are about when
• A brief conversation about the meaning of k in 3.2.3

Build

• James M’s glorious rake task
• Turns out you do have to declare out twice in the HDL
• TextMate beats Jamie’s Vim on this round
• For addressing, we refer back to our n-way (de)muxes from chapter 1
• We use the 3 most significant address bits in our RAM64 to select the RAM8 then we pass the rest
  • Comparable with paging
  • Tree-like approach as smaller chunks of the address are sent down branches
• RAM16K isn’t as big a jump in size as the previous ones, tripped us up
• In x86 we read a word of 32 bits then take a byte out of that
  • The minimum addressable size is 32 bits
  • Boundary alignments are important for performance (to explore)