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+[\<- Decoders and register files](23.md)
+
+---
+
+# Tri-state buffers, RAM
+
+## Tri-state buffer concept and usage for 2:1 mux
+
+### Active outputs
+
+- AND/OR/INV gates actively drive a 0 or 1
+- We can't connect the outputs of two gates because they may want to drive different values
+	- This would be bad (electrically):
+
+![diagram](24.1.png)
+
+### Tri-state Buffer/Driver
+
+- When not enabled, it is no "driving" the output
+- Allows connecting multiple outputs
+	- As long as only one is ever enabled
+
+![diagram](24.2.png)
+
+- Z symbol reflects no active value on the wire (since when e=0, the wire becomes electrically disconnected)
+
+### Muxing with tri-state buffers
+
+- Usage of tri-state bufferis really a form of muxing
+	- The input to the enabled buffer is selected to pass thru to the output
+- A 2:1 mux using tri-state buffers:
+
+![diagram](24.3.png)
+
+---
+
+## Using tri-state buffers for large, distributed muxes
+
+### 1-hot buffer enables
+
+- The key is that only 1 enable can be asserted at a time
+- With four choices and 2 selects => decode
+
+![diagram](24.4.png)
+
+### A distributed mux
+
+- There are four choices here, but we don't have to pull the four inputs into a localized circuit; the buffers can be "far" away
+
+![diagram](24.5.png)
+
+---
+
+## The abstraction of memory
+
+### Memory
+
+- A large number of addressable locations
+	- Just like a register file, but typically many order of magnitude larger
+	- Also like a register file, the number of locations is independent of the data width
+- The address is an encoded value specifying the desired location
+- The structure of the memory may lead to multi-level decoding of the address
+
+### Memory as an abstraction
+
+- Systems with memory are usually fairly complex, so memory is typically represented as an abstraction
+	- Data buses and control signals can differ
+	- M address bits => 2^M locations
+
+![diagram](24.6.png)
+
+---
+
+## An array of SRAM cells
+
+### SRAM cell
+
+- Same feedback loop as latch
+	- By contrast, DRAM uses just a capacitor
+- "Pass" transistors to read and write
+	- Similar to tri-state buffer
+
+![diagram](24.7.png)
+
+### A 2x2 array of cells
+
+- Typically connected in a regular pattern
+	- Cells in a row share the same select line
+	- Cells in a column share the same data line
+
+![diagram](24.8.png)
+
+### A block of SRAM
+
+- Address goes thru row decoder (1-hot)
+
+![diagram](24.9.png)
+
+---
+
+## Multi-level addressing of memory
+
+### Two-level addressing
+
+- For larger memory structures, it becomes inefficient to have just a single column of cells
+- Consider an array of 1024 addressable locations, each of which is just 1-bit of data
+	- 32 rows of 32 columns yields 1024 bits
+	- Break 10-bit addresses into 5 bits for selecting the rows, and 5 bits to select the column
+	- Similar to hotel room numbers
+		- Row is like the floor, column is like the room
+
+### A Memory Chip (1Kx4bit)
+
+- Each sub-array stores one of the 4 bits
+
+![diagram](24.10.png)
+
+### More on address decoding
+
+- Previous example showed break down into rows and columns
+- Some memory chips are banked
+	- Subset of address bits used to select bank
+- In general, full address can be broken down into fields that enumerate/select an instance of a structure
+	- E.g., 4 banks \* 128 rows \* 16 columns
+	- How many address bits for each and in total?
+	- Deconstruct address 0x14B7