Question

A computer uses direct-mapped cache with four16-bit words, and each word has an associ-ated13-bit tag. Consider the following loop (three instructions) in a program. Before the loop,the values in registersR0,R1,R2are 0, 054E, and 2 respectively. Consider that instructions arealready in separate cache memory. (See Table1.)The loop starts at location LOOP=02EC.LOOPAdd(R1)+,R0;DCRR2; BR>0LOOPShow the content of cache at the end of each pass of this loop if direct mapping cache is used.Compute hit rate.

Answer 1

Answer:

Consider the following loop in a program where all instructions and operands are 16 bits long.

Loop Add (R1) + R0

Decrement R2

BNE LOOP

• Assume that, before this loop is entered, registers R0, R1, and R2 contain 0, 054E, and 3, respectively.
• Also assume that the main memory contains the data shown in Fig 1, where all entries are given in hexadecimal notation. The loop starts at location LOOP = 02EC.

• In the rst pass through the loop, the Add instruction is stored at address 4 in the cache, and its operand (A03C) at address 6.
• Then the operand is overwritten by the Decrement instruction.
• The BNE instruction is stored at address 0. In the second pass, the value 05D9 overwrites the BNE instruction, then BNE is read from the main memory and again stored in location 0.
• The contents of the cache, the number of words read from the main memory and from the cache, and the execution time for each pass are as shown below.

• All three instructions are stored in the cache after the rst pass, and they remain in place during subsequent passes.
• In this case, there is a total of 6 read operations from the main memory and 6 from the cache. Execution time is 66 τ .
• Instructions and data are best stored in separate caches to avoid the data overwriting instructions.