How to write a correct code to do 2 writes to an array on sa

W

Weng Tianxiang

Guest
Hi,

Here is a code segment showing 2 methods doing 2 writes to an array with 2 different addresses on the same cycle:

1.
p1: process(CLK) is
begin
if CLK'event and CLK = '1' then
if C1 then
An_Array(a) <= D1;
end if;

-- I know a /= b
-- do I need to inform VHDL compiler of 2 different addresses?
if C2 then
An_Array(b) <= D2;
end if;
end if;
end process;

2.
p2: process(CLK) is
begin
if CLK'event and CLK = '1' then
case C1 & C2 is
when "10" =>
An_Array(a) <= D1;

when "01" =>
An_Array(b) <= D2;

when "11" =>
-- I know a /= b
-- do I need to inform VHDL compiler of 2 different addresses?
An_Array(a) <= D1;
An_Array(b) <= D2;

when others =>
null;
end case;
end if;
end process;

I think it is no problem with a simulator.

Thank you.

Weng
 
On Tuesday, September 24, 2019 at 2:49:14 PM UTC-4, Weng Tianxiang wrote:
Hi,

Here is a code segment showing 2 methods doing 2 writes to an array with 2 different addresses on the same cycle:

1.
p1: process(CLK) is
begin
if CLK'event and CLK = '1' then
if C1 then
An_Array(a) <= D1;
end if;

-- I know a /= b
-- do I need to inform VHDL compiler of 2 different addresses?
if C2 then
An_Array(b) <= D2;
end if;
end if;
end process;
Click to expand...

I think the short answer is YES, the compiler has no way of knowing you are writing to different array elements. More importantly what hardware are you expecting this to synthesize? Simulation is one thing, but I don't know this sort of construct is clear enough to produce any particular hardware. Also, are you targeting an FPGA or a custom chip? I expect there to potentially be a difference.


2.
p2: process(CLK) is
begin
if CLK'event and CLK = '1' then
case C1 & C2 is
when "10" =
An_Array(a) <= D1;

when "01" =
An_Array(b) <= D2;

when "11" =
-- I know a /= b
-- do I need to inform VHDL compiler of 2 different addresses?
An_Array(a) <= D1;
An_Array(b) <= D2;

when others =
null;
end case;
end if;
end process;

I think it is no problem with a simulator.
Click to expand...

Again, the same issue. I believe this code is equivalent to the first example. The assignment statements tend to generate drivers and you may get two drivers on the address input to the array. I expect you are intending to generate a dual port RAM with two separate inputs for address and data with separate write capability. In FPGAs I use the sample code provided by the vendor to infer RAMs. Even if I use my own code, I write the RAM code to be an independent module and then instantiate that.

So what is your target?

--

Rick C.

- Get 2,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209
 
On Wednesday, September 25, 2019 at 9:00:37 AM UTC-7, Rick C wrote:
On Tuesday, September 24, 2019 at 2:49:14 PM UTC-4, Weng Tianxiang wrote:
Hi,

Here is a code segment showing 2 methods doing 2 writes to an array with 2 different addresses on the same cycle:

1.
p1: process(CLK) is
begin
if CLK'event and CLK = '1' then
if C1 then
An_Array(a) <= D1;
end if;

-- I know a /= b
-- do I need to inform VHDL compiler of 2 different addresses?
if C2 then
An_Array(b) <= D2;
end if;
end if;
end process;

I think the short answer is YES, the compiler has no way of knowing you are writing to different array elements. More importantly what hardware are you expecting this to synthesize? Simulation is one thing, but I don't know this sort of construct is clear enough to produce any particular hardware. Also, are you targeting an FPGA or a custom chip? I expect there to potentially be a difference.


2.
p2: process(CLK) is
begin
if CLK'event and CLK = '1' then
case C1 & C2 is
when "10" =
An_Array(a) <= D1;

when "01" =
An_Array(b) <= D2;

when "11" =
-- I know a /= b
-- do I need to inform VHDL compiler of 2 different addresses?
An_Array(a) <= D1;
An_Array(b) <= D2;

when others =
null;
end case;
end if;
end process;

I think it is no problem with a simulator.

Again, the same issue. I believe this code is equivalent to the first example. The assignment statements tend to generate drivers and you may get two drivers on the address input to the array. I expect you are intending to generate a dual port RAM with two separate inputs for address and data with separate write capability. In FPGAs I use the sample code provided by the vendor to infer RAMs. Even if I use my own code, I write the RAM code to be an independent module and then instantiate that.

So what is your target?

--

Rick C.

- Get 2,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209
Click to expand...

Hi Rick,

Thank you for your answer.

Actually I am writing code not targeting any FPGA manufactures, neither Altera nor Xilinx, I just want that the code can be simulated by Mentor simulator to prove that my algorithm on a universal subject is working IN THEORY: with 2 write ports for an array, everything is working. By doing so it will save me a lot of energy, cost and time.

Translating 2 writes at 2 different addresses for an array in code to a 2 write port memory is considered as a mature technology like to implement an 64-bit adder if one decides to select a FPGA chip to implement.

I think based on the definition in a sequential process with or without clock, my code is working: if conditions C1 = '1' and C2 = '1', first the simulator by definition executes An_Array(a) <= D1; then An_Array(b) <= D2; because a /= b, so An_Array gets its 2 writes without any problem in simulation.

I know if I want the code to be executed on an Altera chip or a Xilinx chip, I have to use one of multiple possible 2 writes memory mechanisms to implement the code. But at this stage, I don't want to spend time doing it.

At this stage, I think I don't have to put consideration to it.

Weng
 
On Wednesday, September 25, 2019 at 10:47:04 AM UTC-7, KJ wrote:
On Wednesday, September 25, 2019 at 1:31:35 PM UTC-4, Weng Tianxiang wrote:
On Wednesday, September 25, 2019 at 9:00:37 AM UTC-7, Rick C wrote:
On Tuesday, September 24, 2019 at 2:49:14 PM UTC-4, Weng Tianxiang

I just want that the code can be simulated by Mentor simulator to prove that my algorithm on a universal subject is working IN THEORY: with 2 write ports for an array, everything is working. By doing so it will save me a lot of energy, cost and time.

So, you want someone else to run a VHDL simulator for you in order to save you a lot of energy, cost and time? Ummm...why, are you being lazy? I can pretty much guarantee that any VHDL simulator will properly execute the code that you've written per the VHDL language standard. Whether that code performs the function that you would like it for your "algorithm on a universal subject" is up to you to decide. Best of luck on your new endeavors.

Kevin Jennings
Click to expand...

Hi KJ,

I never said that: "So, you want someone else to run a VHDL simulator for you in order to save you a lot of energy, cost and time?"

I will do the simulation by myself to determine if the algorithm is correct..

And I will let others to test if the code implementation on either a Xilinx chip or an Altera chip is working.

A 2 write port memory is a mature technique and I don't have to spend a lot of time to do it myself.

Thank you.

Weng
 
On Wednesday, September 25, 2019 at 1:31:35 PM UTC-4, Weng Tianxiang wrote:
On Wednesday, September 25, 2019 at 9:00:37 AM UTC-7, Rick C wrote:
On Tuesday, September 24, 2019 at 2:49:14 PM UTC-4, Weng Tianxiang

I just want that the code can be simulated by Mentor simulator to prove that my algorithm on a universal subject is working IN THEORY: with 2 write ports for an array, everything is working. By doing so it will save me a lot of energy, cost and time.
Click to expand...

So, you want someone else to run a VHDL simulator for you in order to save you a lot of energy, cost and time? Ummm...why, are you being lazy? I can pretty much guarantee that any VHDL simulator will properly execute the code that you've written per the VHDL language standard. Whether that code performs the function that you would like it for your "algorithm on a universal subject" is up to you to decide. Best of luck on your new endeavors.

Kevin Jennings
 
On Wednesday, September 25, 2019 at 2:07:45 PM UTC-4, Weng Tianxiang wrote:
I never said that: "So, you want someone else to run a VHDL simulator for you in order to save you a lot of energy, cost and time?"

I paraphrased slightly, but yes you did say that. Read your original post which I quoted you on.
Click to expand...

> I will do the simulation by myself to determine if the algorithm is correct.

Cool.
And I will let others to test if the code implementation on either a Xilinx chip or an Altera chip is working.

Beyond a superficial, quick code review, why would anyone want to do that?
Click to expand...

A 2 write port memory is a mature technique and I don't have to spend a lot of time to do it myself.

And yet here you are, writing up your own code for two write port memory which you say "is a mature technique". That's pretty much the definition of re-inventing the wheel.
Click to expand...

Kevin Jennings
 
On Wednesday, September 25, 2019 at 12:34:13 PM UTC-7, KJ wrote:
On Wednesday, September 25, 2019 at 2:07:45 PM UTC-4, Weng Tianxiang wrote:

I never said that: "So, you want someone else to run a VHDL simulator for you in order to save you a lot of energy, cost and time?"

I paraphrased slightly, but yes you did say that. Read your original post which I quoted you on.

I will do the simulation by myself to determine if the algorithm is correct.

Cool.

And I will let others to test if the code implementation on either a Xilinx chip or an Altera chip is working.

Beyond a superficial, quick code review, why would anyone want to do that?

A 2 write port memory is a mature technique and I don't have to spend a lot of time to do it myself.

And yet here you are, writing up your own code for two write port memory which you say "is a mature technique". That's pretty much the definition of re-inventing the wheel.

Kevin Jennings
Click to expand...

KJ,

I prefer for VHDL grammar to introduce a new statement, specifying an if statement is a second write to an array. Here is a code example on how to introduce such statement:

Here is a code segment showing method, doing 2 writes to an array with 2 different addresses on the same cycle:

p1: process(CLK) is
begin
if CLK'event and CLK = '1' then
if C1 then
An_Array(a) <= D1;
end if;

-- "IF_2" is a new keyword which introduces a second write to an array in its full range, including all "else", "elsif" parts. And "if_2" keyword can only be used in a clocked process.

if_2 C2 then
An_Array(b) <= D2;
end if;
end if;
end process;

Using the new suggested keyword "if_2" in VHDL, everybody would like it, not having repeatedly to write a 2-write-port memory for different FPGA chip.

In my design there are more than 10 arrays need 2-write port memory.

Weng
 
On 25/09/2019 22:22, Weng Tianxiang wrote:
On Wednesday, September 25, 2019 at 12:34:13 PM UTC-7, KJ wrote:
On Wednesday, September 25, 2019 at 2:07:45 PM UTC-4, Weng Tianxiang wrote:
...
p1: process(CLK) is
begin
if CLK'event and CLK = '1' then
if C1 then
An_Array(a) <= D1;
end if;

-- "IF_2" is a new keyword which introduces a second write to an array in its full range, including all "else", "elsif" parts. And "if_2" keyword can only be used in a clocked process.

if_2 C2 then
An_Array(b) <= D2;
end if;
end if;
end process;

Using the new suggested keyword "if_2" in VHDL, everybody would like it, not having repeatedly to write a 2-write-port memory for different FPGA chip.

In my design there are more than 10 arrays need 2-write port memory.

Weng
Click to expand...

Hi Weng,

Give it up, your are flogging a dead horse, no new language constructs
are requires as we already have the correct simulation and synthesis
models for decades.

If you have lots of dual port memories to connect use a Generate statement.

Regards,
Hans
www.ht-lab.com
 
On Wednesday, September 25, 2019 at 5:22:34 PM UTC-4, Weng Tianxiang wrote:
On Wednesday, September 25, 2019 at 12:34:13 PM UTC-7, KJ wrote:
On Wednesday, September 25, 2019 at 2:07:45 PM UTC-4, Weng Tianxiang wrote:

I prefer for VHDL grammar to introduce a new statement, specifying an if statement is a second write to an array.
'I prefer' is not justification. One needs to provide objective, measurable improvement. 'I prefer' doesn't do it.
Click to expand...

Here is a code example on how to introduce such statement:
Yet your example by one measure shows how your new method is worse. Rather than simply typing 'if', now one types 'if_2'. How is that better? The same simulation and synthesis results would occur but now any designer is typing more to achieve the same...sort of unproductive don't you think? The language gets more complex and produces no objective benefit to anyone...not the sort of change that benefits.
Click to expand...

Using the new suggested keyword "if_2" in VHDL, everybody would like it, not having repeatedly to write a 2-write-port memory for different FPGA chip.

Everybody? How about anybody besides you? Who are these people that "repeatedly to write a 2-write-port memory" rather than writing it once? Those people need to learn how design should be done.
Click to expand...

In my design there are more than 10 arrays need 2-write port memory.

This statement simply demonstrates your total lack of ability in design. If you're writing the same thing 10 times you're doing it the wrong way.
Click to expand...

Kevin Jennings
 
On Thursday, September 26, 2019 at 2:50:48 AM UTC-7, HT-Lab wrote:
On 25/09/2019 22:22, Weng Tianxiang wrote:
On Wednesday, September 25, 2019 at 12:34:13 PM UTC-7, KJ wrote:
On Wednesday, September 25, 2019 at 2:07:45 PM UTC-4, Weng Tianxiang wrote:
..
p1: process(CLK) is
begin
if CLK'event and CLK = '1' then
if C1 then
An_Array(a) <= D1;
end if;

-- "IF_2" is a new keyword which introduces a second write to an array in its full range, including all "else", "elsif" parts. And "if_2" keyword can only be used in a clocked process.

if_2 C2 then
An_Array(b) <= D2;
end if;
end if;
end process;

Using the new suggested keyword "if_2" in VHDL, everybody would like it, not having repeatedly to write a 2-write-port memory for different FPGA chip.

In my design there are more than 10 arrays need 2-write port memory.

Weng


Hi Weng,

Give it up, your are flogging a dead horse, no new language constructs
are requires as we already have the correct simulation and synthesis
models for decades.

If you have lots of dual port memories to connect use a Generate statement.

Regards,
Hans
www.ht-lab.com
Click to expand...

Hans,

Here is an excellent paper dealing with the situation: efficient multi-port memory with FPGA, http://www.eecg.toronto.edu/~steffan/papers/laforest_fpga10.pdf.

I appreciate the paper very much. I know there is a solution to 2-write memory, but simple "if_2" is a simple solution to inferring to the method.

Weng
 
On Thursday, September 26, 2019 at 9:36:46 AM UTC-4, Weng Tianxiang wrote:
On Thursday, September 26, 2019 at 2:50:48 AM UTC-7, HT-Lab wrote:
On 25/09/2019 22:22, Weng Tianxiang wrote:
On Wednesday, September 25, 2019 at 12:34:13 PM UTC-7, KJ wrote:
On Wednesday, September 25, 2019 at 2:07:45 PM UTC-4, Weng Tianxiang wrote:
..
p1: process(CLK) is
begin
if CLK'event and CLK = '1' then
if C1 then
An_Array(a) <= D1;
end if;

-- "IF_2" is a new keyword which introduces a second write to an array in its full range, including all "else", "elsif" parts. And "if_2" keyword can only be used in a clocked process.

if_2 C2 then
An_Array(b) <= D2;
end if;
end if;
end process;

Using the new suggested keyword "if_2" in VHDL, everybody would like it, not having repeatedly to write a 2-write-port memory for different FPGA chip.

In my design there are more than 10 arrays need 2-write port memory.

Weng


Hi Weng,

Give it up, your are flogging a dead horse, no new language constructs
are requires as we already have the correct simulation and synthesis
models for decades.

If you have lots of dual port memories to connect use a Generate statement.

Regards,
Hans
www.ht-lab.com

Hans,

Here is an excellent paper dealing with the situation: efficient multi-port memory with FPGA, http://www.eecg.toronto.edu/~steffan/papers/laforest_fpga10.pdf.

I appreciate the paper very much. I know there is a solution to 2-write memory, but simple "if_2" is a simple solution to inferring to the method.

Weng
Click to expand...

You have provided a keyword, "if_2" and a code example which is not logically correct. I don't see how your keyword solves anything. First, you need to define exactly what the keyword does, which I assume is the same as "if". Then you need to use examples which are logically correct. Then perhaps you can explain why your new keyword accomplishes something that the existing keyword doesn't do already.

It sure seems like you are trying to solve a problem that doesn't exist. Certainly your initial premise that dual write port RAMs must be instantiated is not correct. Dual write port RAMs can be inferred as shown in the info I linked to in the other thread.

--

Rick C.

+ Get 2,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209
 
On 26/09/2019 14:36, Weng Tianxiang wrote:
On Thursday, September 26, 2019 at 2:50:48 AM UTC-7, HT-Lab wrote:
On 25/09/2019 22:22, Weng Tianxiang wrote:
...

Hi Weng,

Give it up, your are flogging a dead horse, no new language constructs
are requires as we already have the correct simulation and synthesis
models for decades.

If you have lots of dual port memories to connect use a Generate statement.

Regards,
Hans
www.ht-lab.com

Hans,

Here is an excellent paper dealing with the situation: efficient multi-port memory with FPGA, http://www.eecg.toronto.edu/~steffan/papers/laforest_fpga10.pdf.

I appreciate the paper very much. I know there is a solution to 2-write memory, but simple "if_2" is a simple solution to inferring to the method.

Weng
Click to expand...

Hi Weng,

Yes I know that paper quite well and have used the XOR variant in one of
my x86 processors (4W8R). I had no problems implementing it, the
standard method is simple and straightforward.

As others have told you multi port memories are well established and
understood in our community, there is no need for a special keyword.

Regards,
Hans
www.ht-lab.com
 
On Thursday, September 26, 2019 at 9:36:46 AM UTC-4, Weng Tianxiang wrote:

Here is an excellent paper dealing with the situation: efficient multi-port memory with FPGA, http://www.eecg.toronto.edu/~steffan/papers/laforest_fpga10.pdf.

I appreciate the paper very much. I know there is a solution to 2-write memory, but simple "if_2" is a simple solution to inferring to the method.
Click to expand...

If_2 is not the solution to inferring the method by LaForest and Steffan. In a nutshell, what they describe is the bolded text in section 5 (LVT-Based Multiported memories) where it clearly says "Our approach comprises
banks of replicated block RAMs where a mechanism of indirection...". They are not claiming to implement something that implements the two write port hardware description that you presented. There would still need to be code written to implement the LVT approach in order to describe that LVT design.. Once that code is written, one would have a multi-port design that one could reuse but it would use more logic/memory resources than just a memory.

If you think the LVT based approach is worth using for your design, then write the code to implement an LVT based memory design. Trying to propose 'if_2' as being some way to implement LVT (or some other alternative approach) is quite naive on your part.

Kevin Jennings
 
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