
246 THE x86 MICROPROCESSORS
KEY POINTS OF THIS CHAPTER
Memory mapped I/O causes I/O space to encroach on the memory address space of the
processor.
Active low control signals prevent accidental triggering by noise voltages or open connections.
74LS138 is a very popular 3 to 8 decoder.
The number of locations in a memory chip is 2
N
where N is the number of address lines of the chip.
Memory is always organized as ‘banks’.
The BHE signal goes low when the addressed byte is in the upper bank of memory.
The LSB of the address bus i.e., A
0
is used to enable the low bank of memory.
Memory chips have tri-state bu ers and latches inside the chip ...