RTL Components

Bus

• Group of $n\ge 1$ signals
• 
• At RT-level, busses are used to connect system components
  • 
  • Advantages compared to point-to-point
    • Low hardware overhead: all components use the same bus
    • scalability: easy to add new components without needing to redesign
    • Easy broadcasting: One component writes, all others read
  • Drawbacks compared to point-to-point
    • Sequential communication
      • Has a protocol for transmitting signals serially
    • Single point of failure
  • Technology must allow multiple outputs on a single line (“tri-state”)
  • We have very few of these busses on modern computers
    • Now most things communicate in parallel

N-Bit (Word) Gate

• Grouping of $n\ge 1$ independent gates
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Multiplexer

• Connects one of $n$ inputs to a single output at a time
• Also known as a data selector
  • $n$ data inputs
  • 1 data output
  • $k$ select input ($n\le 2^k$)
• Logic function
  • $z={\sum }_{j=0}^{n-1}{m}_j(a_{k-1},\dots ,a_0)x_j$
  • $m_j(a_{k-1},\dots ,a_0)$ is the $j^{th}$ minterm of $k$ variables
  • $a_{k-1},\dots ,a_0$ addresses the input data
• Extensions
  • Selecting a word
    • $z_i={\sum }_{j=0}^{n-1}{m}_j\cdot x_{j,l}\forall 0\dots m-1$
  • Multiple Selections
    • $m\lceil {\log }_{2}n\rceil$ selector inputs
    • Written (IN):(OUT) MUX
• How if/else is implemented
• One of the more expensive components in hardware

Demultipexer

• Connects 1 inputs to one of n outputs
• The inverse of a multiplexor
• Logic function: $z_j=m_j(a_{k-1}\dots a_0)\cdot x$

Encoder

• A logic block that outputs a unique binary number for each input
• $2^n$ inputs, $n$ outputs
• Have an additional output (input active) used to recognize cases where all inputs are 0
• Priority Encoder
  • Many inputs can be 1 simultaneously
  • The output is the binary code for the biggest index $i$ where $x_i=1$
  • All inputs have priority, where the higher the index, the higher the priority

Decoder

• A logic block that assigns a single output for each possible combination of inputs
• $n$ inputs, $2^n$ outputs
• Application: selecting the word to read in Memory