Memory: chip enable

Need to generate signals to determine which addresses to access in memory

Logic for chip enable

CPU can generate 3 control signals:

B which indicates that the CPU wants to access a byte

H which indicates that the CPU wants to access a halfword

W which indicates that the CPU wants to access a word

Which chips are enabled?

B: 1 chip

H: 2 chips

W: all 4 chips

Address patterns:

addresses on chip 00 end in 00 (divisible by 4)

addresses on chip 01 end in 01 (congruent to 1 mod 4)

addresses on chip 10 end in 10 (congruent to 2 mod 4)

addresses on chip 11 end in 11 (congruent to 3 mod 4)

Logic for chip 00:

If W = 1, then all four chips are enabled

If B = 1, select chip 00 when A1A0 = 00

If H = 1, select Chip 00 if A1 = 0 (already know that A0 = 0)

Boolean expression:

CE = W + (H * A1') + (B * A1'A0')

Notice address bits A9-0:

To get M[i], access index i / 4

Same as shifting to the right by 2 bits, or accessing bits 11-2 from the address bus

Chip 10:

CE = W + (H * A1) + (B * A1A0')

Invalid addresses:

we assume that the other 20 bits of the address are all 0