Description
Problem Statement: MIPS simulator with DRAM timing model
A basic MIPS interpreter handling a subset of the ISA is developed by you in Assignment 3. The
objective is to enhance it with two new features, making it a MIPS simulator.
1. Develop a model for the Main Memory and integrate into the basic interpreter.
2. The memory access should be non-blocking (subsequent instructions don’t always wait for
the previous instructions to complete).
Input:
1. MIPS assembly language program (as text file, NOT machine instructions).
2. DRAM timing values ROW_ACCESS_DELAY and COL_ACCESS_DELAY in cycles (as
command line arguments). Typical values could be 10 cycles and 2 cycles
respectively.
Main Memory Timing Characteristics
You will model the access times corresponding to a simplified version of the Dynamic Random
Access Memory (DRAM). Such a memory can be thought of as a 2-dimensional array of ROWS and
COLUMNS (see picture below). Our memory of size 220 bytes consists of 1024 rows, with 1024
columns (bytes) in each row. A single Read/Write in a lw/sw instruction corresponds to accessing
a 32-bit (4 byte) value from the DRAM. Within a row, a given piece of data is located at a column
offset, so a memory address could be thought of as consisting of a ROW ADDRESS and COLUMN
ADDRESS.
A READ operation (corresponding to an lw instruction) works as follows.
1. Activate the ROW corresponding to this address by COPYING the row to a ROW BUFFER at
the bottom of the structure. Time required for this operation: ROW_ACCESS_DELAY.
2. Copy the data at the column offset from the row buffer to the REGISTER. Time for this
operation: COL_ACCESS_DELAY.
For subsequent READ operations, we first check whether the corresponding row is already located
in the ROW BUFFER. If yes, then just copy the data at the column offset to the data bus after time
COL_ACCESS_DELAY (no need for copying the row into the row buffer, since it is already present).
However, if the row is different from the one currently located in the row buffer, then:
1. First copy the row buffer back to its row (Time for this operation: ROW_ACCESS_DELAY).
2. Access the data from a new row going through the READ operation protocol described
earlier.
A WRITE operation (corresponding to an sw instruction) works as follows. If the row is present in
the row buffer, update the row buffer data (at the column offset) in time COL_ACCESS_DELAY. If
the row is not present, then:
1. Copy the row buffer back (Time for this operation: ROW_ACCESS_DELAY).
2. Copy the new row to the row buffer (Time for this operation: ROW_ACCESS_DELAY).
3. Update the data in the row buffer in time COL_ACCESS_DELAY.
Non-blocking Memory Access
In implementing the DRAM timing above, we notice that the processor slows down significantly
because of DRAM accesses. Try reducing the execution time by observing that it may be possible to
Row Buffer
1024 bytes
1024
rows
Row
Data bus
Address
Data location Row
address
Column
address
copy
DRAM Illustration
Processor
execute the next instruction (or next few instructions) even if a lw/sw instruction has not
completed. When is it safe to do so? Explain, and implement this feature in your MIPS simulator.
Note: Try to first implement a relatively simple solution before trying out more complex ideas.
Output:
1. At every clock cycle, print the clock cycle number and all activity in that cycle, such as:
a. Print executed instruction
b. Modified registers, if any (register number and new value)
c. Modified memory locations, if any (memory location and new value (4 bytes))
d. Activity on the DRAM, if any (memory location, row buffer updates)
2. After execution completes, print the relevant statistics such as:
a. Total execution time in clock cycles
b. Number of row buffer updates
Assume the following:
1. Instructions may be in memory, but are not fetched from DRAM (consider instruction
access delay to be zero). Only lw/sw instructions result in DRAM accesses.
2. Use the same architectural and ISA assumptions as in Assignment 3.
Marks Distribution:
1. Part 1 (DRAM implementation): 10 marks
2. Part 2 (Non-blocking memory): 10 marks
Test cases: Please refer to the following:
1. Presentation having the detailed description of sample test cases.
2. Test cases uploaded in a zip folder.
