Counters and Shift Registers

Building a computer? Things we need:

memory within Fast memory (RAM) Persistent the CPU – program and data memory

= banks (Disks)

of flip- flops Instruction Pointer Central processing Motherboard (counter) unit (CPU) registers IP full adder + Screen shift ALU operations Keyboard

Input / Output accel

2 Channels cards Mouse

REGISTERS

Fast RAM (uses transistor states)

• Uses clocked flip-flops
• Inside the CPU chip
• Limited number of them (cause it's hardware)
• Let's build one...

D-TYPE FLIP-FLOP

• Set data to high – Q goes high on next clock pulse. Stays high.
• Set presets Q to high when pulled high
• Reset clears Q when pulled high

• In Logisim we can also leave PR' and CLR' disconnected.

D-FLIP-FLOPS AS A REGISTER OR LATCH

A register (many bits) or latch (usually one bit) can be made Input before the clock Information 1 0 1 1 up from a series of D-Flip-Flops latched driven by a common clock. now D D D D

The transfer from the D side to the Clock Q Q Q Q Q side for all D flip flops occurs 1 0 1 1 simultaneously as this clock Output after the clock changes. This arrangement is found in CPU registers http://www.electronics-tutorials.ws/sequential/seq_4.html

6 COS10004 Computer Systems 6

8 BIT REGISTER

little-endian

Little Endian

BIG ENDIAN AND LITTLE ENDIAN DATA FORMAT

• Technical definition for the order of bytes:
• Big Endian:
• The most signficant byte resides in the smallest memory address
• Little Endian:
• The least significant byte resides in the smallest memory address

BIT ENDIANNESS

• Bits generally don’t have an address, so definitions refer to the positional order of bits
• Big Endian:
• The most significant bit comes first
• Little Endian:
• The least significant bit comes first
• This matters for interpreting the value of a bit string (especially if bits are received as a serial stream!)
• Eg What is 1011 in decimal?
• 1110 (Big endian), or 1310 (Little endian) 9

8 BIT REGISTER

little-endian

Little Endian

Big Endian

SUMMARY

• D Flip Flops generally form the building blocks of registers
• Registers:
• Allow us to store bit strings that represent data
• Reside close to the CPU, allowing fast and easy access
• Bit Endianness determines how we interpret bit strings:
• Big Endian: most significant bit comes first
• Little Endian: least significant bit comes first
• Next Lecture: ripple counters with JK Flip Flops