In an instruction execution pipeline, the earliest that the data TLB (Translation Lookaside Buffer) can be accessed is:
A. before effective address calculation has started
B. during effective address calculation
C. after effective address calculation has completed
D. after data cache lookup has completed
GATE 2008 · Computer Organization and Architecture · Virtual Memory · medium
Answer: The data TLB can be accessed at the earliest after effective address calculation has completed. Answer: C.
Identify what the data TLB needs as input: The TLB maps virtual page numbers to physical frame numbers. To perform this lookup, the virtual page number (derived from the effective virtual address) must be available. Without the effective address, the TLB has no input to look up.
Determine ordering with respect to data cache: The data cache (or main memory) is accessed using the PHYSICAL address, which comes from the TLB output. Therefore, TLB must be accessed BEFORE the data cache. This rules out option D (after data cache lookup). The TLB lookup produces the physical address that the data cache needs.
Determine correct answer: Pipeline ordering for data memory access:
1. Effective Address Calculation -> produces virtual address
2. TLB Lookup (EARLIEST possible) -> translates virtual to physical
3. Data Cache Lookup -> uses physical address from TLB
The TLB can be accessed earliest at step 2, which is after effective address calculation has completed.
Correct answer: C.