FIFO Full and Empty Explanation

Introduction

• Discussing FIFO (First In, First Out) conditions: full and empty.
• Example with four locations: 00, 01, 10, 11.

Key Concepts

Overflow and Underflow

• Overflow: Occurs when writing to FIFO without reading, leading to data loss.
• Underflow: Occurs when reading from FIFO when no data is available, leading to junk data.

Signals for Control

• Full Signal: Indicates FIFO is full; prevents further writes.
  • Generated when all four locations have data.
• Empty Signal: Indicates FIFO is empty; prevents reads.
  • Generated when there is no data to read.

Pointers in FIFO

• Write Pointer (WPtr): Indicates the location where the next write will occur.
• Read Pointer (RPtr): Indicates the location where the next read will occur.

Pointer Behavior

• Initially, both pointers start at 00.
• WPtr increments with each write, and RPtr increments with each read.
• Condition for full: WPtr equals RPtr after writing four times.
• Condition for empty: WPtr equals RPtr with no data.

Extra Bit Requirement

• Need an extra bit to represent FIFO states due to pointer comparison.
  • With four locations, two bits alone are insufficient to distinguish between full and empty states.
  • With three bits, the most significant bit (MSB) helps differentiate between full and empty conditions.

Full Condition Example

• If five data pieces are written and one is read, it results in four filled locations.
  • Comparing pointers shows MSB is not equal while remaining bits are equal, generating the full condition.

Empty Condition Example

• If three writes and then three reads occur, both pointers eventually match, indicating empty status.
  • Condition for empty: WPtr equals RPtr.

Conclusion

• FIFO control is essential to prevent overflow and underflow.
• Proper use of pointers and signals ensures data integrity in FIFO operations.