V22.0201.002, Machine Organization I
Fall 1997

NOTES ON KEYBOARD

INTRODUCTION

• INT 16h is used for keyboard interrupts.
• Read up about the keyboard buffer in text book.
• Addresses of the keyboard buffer (from Abel, p.198)

  	ADDRESS		EXPLANATION
  	41Ah		Address of the current head of the buffer
  			(the next position for INT 16h to read a char)
  	41Ch		Address of the current tail of buffer
  			(the next position for INT 16h to store a char)
  	41Eh		Address of the beginning of the keyboard buffer.
  			The buffer contains 16 words (32 bytes), but
  			can be longer.
  	

  The buffer is empty when the head and tail of buffer coincide. Note we have 16 words in the buffer, but only 15 type ahead characters are allowed. This is because if we have 16 words, then we cannot distinguish between buffer full and buffer empty!

KEYBOARD INTERRUPT FUNCTIONS