# Registers and Memory --- CS 130 // 2025-10-20 <!--=====================================================================--> # Review <!-- .slide: data-background="#004477" --> <!--=====================================================================--> ## Sequential Logic - **Sequential** logic circuits have memory + <!-- .element: class="fragment"--> Output depends on both the input and current state of memory + <!-- .element: class="fragment"--> Behavior can change **over time** <!--=====================================================================--> ### Sequential Logic Components - **Clock**: a signal that oscillates between 0 and 1 - <!-- .element: class="fragment"--> **Latch**: 1-bit memory element that is updated when the clock is 1 - <!-- .element: class="fragment"--> **Flip Flop**: 1-bit memory element that is updated when the clock transitions from a 1 to a 0 <!--=====================================================================--> ## Registers - A **register** is a multi-bit memory component - Registers are commonly implemented with an array of D flip flops <!--=====================================================================--> ## Register Files - A **register file** consists of an array of registers that can be read and written to  <!----------------------------------> ## Register Files - **Reading** is implemented with multiplexers  <!----------------------------------> ## Register Files - **Writing** is implemented with a decoder  <!--=====================================================================--> ## Decoder - A **decoder** is another common circuit that has $n$ inputs and $2^n$ outputs - <!-- .element: class="fragment"--> Only one output is a 1 at any given time (one for each possible combination of inputs) - <!-- .element: class="fragment"--> If the input encodes the number 7, then the output bit for 7 is asserted and all others are zeros <!----------------------------------> ## Decoder Truth Table - Below is a 3-bit decoder truth table  <!--=====================================================================--> ## Exercise: Build a register File in Logisim <div class="twocolumn"> <div> - 32 bits - at least 4 registers  <!-- .element width="300px" --> </div> <div>  <!-- .element width="200px" -->  <!-- .element width="200px" -->  <!-- .element width="400px" --> </div> </div> <!--=====================================================================--> # ROM <!-- .slide: data-background="#004477" --> <!--=====================================================================--> ## ROM: Read-Only Memory - Intended only to be read from (though it may be possible to change it in some circumstances) - **Non-volatile**: keeps its value without power - Game Cartidges, CD-ROM, DVD - Firmware for PCs and other devices + contains boot-up instructions  <!-- .element width="200px" -->  <!-- .element width="200px" --> <!--=====================================================================--> # RAM <!-- .slide: data-background="#004477" --> <!--=====================================================================--> ## RAM: Random Access Memory - It takes the same amount of time to access data regardless of where it is  <!-- .element width="400px" --> <!--=====================================================================--> ## Static Random Access Memory - Memory where data is stored *statically* with flip-flops - Faster than DRAM but less dense (lower capacity) and more expensive per bit - As long as it has power, SRAM will keep its value - Usually used for cache memory <!--=====================================================================--> ## Dynamic Random Access Memory - DRAM is the most dense RAM * Implemented with capacitors instead of latches - stores energy in an electrical field - Must be occasionally "refreshed" because the capacitors slowly lose their stored values * This is why it is called "dynamic" - Usually used for large capacity RAM