Introduction
In the realm of modern computing, cache memory plays an indispensable role, bridging the gap between the processor and main memory to ensure faster data access and reduced latency. This article delves into the intricacies of cache memory design, focusing on its implementation using Verilog, a prevalent hardware description language for modeling digital systems.
Understanding Cache Memory Design
Cache memory operates as a high-speed buffer between the processor and the main memory, storing copies of frequently accessed data to expedite future accesses. One common approach to cache design is the use of a FIFO (First In, First Out) structure, where newer data is inserted at one end while older data is removed from the opposite end.
Key Components of Cache Design
👉 Method
Cache design feature methods for finding and updating data. The find method searches for a specific key in the cache and returns whether it’s found along with its corresponding value. The update method allows for modifications to existing key-value pairs in the cache.
Implementation in Verilog
module cache (
input clk,
input reset,
input find,
input [7:0] key,
output reg match_found,
output reg [7:0] value,
input update,
input [7:0] update_key,
input [7:0] update_value
);
localparam words = 8;
logic [(words*16)-1:0] data_vec;
// Find logic
always @(posedge clk) begin
if (reset) begin
match_found <= 0;
value <= 0;
end else begin
match_found <= 0;
value <= 0;
for (int i = words; i > 0; i--) begin
if (find) begin
if (data_vec[(i*16)-1 -:8] == key) begin
match_found <= 1;
value <= data_vec[(i*16)-9 -:8];
end
end
end
end
end
// Update logic
always @(posedge clk) begin
if (reset) begin
for (int i = 1; i < words+1; i++) begin
data_vec[(i*16)-1 -:8] <= i; // some default value
data_vec[(i*16)-9 -:8] <= i+50; // some default value
end
$display("INIT-CACHE is %x", data_vec);
end else begin
if (update) begin
data_vec[(words*16)-1 -:(words*16)] <= {data_vec[((words-1)*16)-1 -:((words-1)*16)], update_key, update_value};
end
end
end
endmodule
Find Logic
- If
resetis asserted,match_foundandvalueare reset to zero. - Otherwise, for each word in the cache:
- If
findis asserted, it checks if the key matches the key stored in the cache (data_vec). - If a match is found,
match_foundis set to 1, and the corresponding value is assigned tovalue.
- If
Update Logic
Another always block sensitive to the positive edge of the clock (clk) is defined. Within this block:
- If
resetis asserted, default values are assigned to the cache data. - Otherwise, if
updateis asserted, the cache data is updated:- The existing cache data is shifted to make space for the new key-value pair (
update_key,update_value).
- The existing cache data is shifted to make space for the new key-value pair (
👉 Steps to Design the testbench
module tb;
// Declare signals
logic find, match_found;
logic [7:0] key, value;
logic [7:0] update_key, update_value;
logic update;
logic clk, reset;
// Instantiate cache module
cache cache_1 (
.clk(clk),
.reset(reset),
.find(find),
.key(key),
.match_found(match_found),
.value(value),
.update(update),
.update_key(update_key),
.update_value(update_value)
);
// Initialize signals
initial begin
reset = 1;
clk = 0;
#101 reset = 0;
end
// Clock generation
always @(clk) begin
#5 clk = ~clk;
end
// Stimulus generation
initial begin
#121;
update_key = 8'h01;
update = 1;
update_value = 100;
#10 $display("UPD-CACHE is %x ", cache_1.data_vec);
find = 0;
#10;
key = 8'h09;
find = 1;
#10 $display("Match for key 0x09 is %d value is %x", match_found, value);
find = 0;
#10 key = 8'h01;
find = 1;
#10 $display("Match for key 0x01 is %d value is %x", match_found, value);
$finish;
end
endmodule👉 Stimulus Generation
- Define an initial block to control the stimulus for the cache module.
- After a delay of 101 time units, de-assert the
resetsignal to initiate cache operation. - Set up an update operation by assigning values to
update_keyandupdate_valueafter a delay of 121 time units. - Wait for 10 time units and then display the updated cache data using
$display. - Simulate a find operation for key
8'h09by settingfindto 1 andkeyto8'h09. Display the match result after a delay. - Simulate another find operation for key
8'h01by changingkeyandfindand display the match result after a delay.
Performance Considerations
Efficiency is paramount in cache design. While the FIFO mechanism employed in our cache design offers simplicity, it may lead to inefficiencies if the cache continuously receives the same value. However, parameterization and optimization strategies can mitigate such drawbacks and enhance cache performance.
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