In VHDL is there a way of limiting the size of a string with a variable value from a process? - vhdl

Question

My goal is to write a string to a file where the size of the string will vary. At the moment I have made the string very large so that there is no overflow but is there a way to make it so that the size of the string is the exact number of characters I'm placing into it? I've tried something like the code below but it gives me an error unknown identifier "address count" I think it is because address count is a variable declared in a process and address count is constantly changing. Is there any way around this?
signal address_map :string (1 to address_count);
many thanks
leo

Answer 1

"My goal is to write a string to a file." Hence, lets just focus on that.
Step 1: reference the file IO packages (recommended to turn on VHDL-2008):
use std.textio.all ;
-- use ieee.std_logic_textio.all ; -- include if not using VHDL-2008
Step 2: Declare your file
file MyFile : TEXT open WRITE_MODE is "MyFile.txt";
Step 3: Create a buffer:
TestProc : process
variable WriteBuf : line ;
begin
write ... -- see step 4
writeline ... -- see step 5
Step 4: Use write to write into the buffer (in the process TestProc):
write(WriteBuf, string'("State = ") ) ; -- Any VHDL version
write(WriteBuf, StateType'image(State)) ;
swrite(WriteBuf, " at time = " ); -- VHDL-2008 simplification
write(WriteBuf, NOW, RIGHT, 12) ;
Step 5: Write the buffer to the file (in the process TestProc):
writeline(MyFile, WriteBuf) ;
Alternate Steps 3-5: Use built-in VHDL Write with to_string:
Write(MyFile, "State = " & to_string(State) &
", Data = " & to_hstring(Data) &
" at time " & to_string(NOW, 1 ns) ) ;
Alternate Steps 1-5: Use OSVVM (see http://osvvm.org) (requires VHDL-2008):
library osvvm ;
use osvvm.transcriptpkg.all ; -- all printing goes to same file
. . .
TestProc : process
begin
TranscriptOpen("./results/test1.txt") ;
Print("State = " & to_string(State) &
", Data = " & to_hstring(Data) &
" at time " & to_string(NOW, 1 ns) ) ;

Answer 2

One hard but flexible solution is to use dynamic allocation features of VHDL (copied from ADA).
You have to use an access of string (it is roughly like a "pointer to a string" in C)
type line is access string;
you event don't have to do it because line is already declared in std.textio package.
Ok, the problem next is that you can't use an access type for a signal, so you have to use a shared variable:
shared variable address_map: line;
And finally you have to allocate, read and write to this line:
--Example in a function/procedure/process:
--free a previously allocated string:
if address_map /= NULL then
deallocate(address_map);
end if;
--allocate a new string:
address_map:=new string (1 to address_count);
address_map(1 to 3):="xyz";
--we have here:
-- address_map(1)='y'
-- address_map(2 to 3)="yz"
-- address_map.all = "xyz"
Notice the use of new/deallocate (like malloc/free in C or free/delete in C++).
It is not easy to handle this kind of code, I recommend you to read the documentation of VHDL keywords "new", "deallocate" and "access" (easily found with your favorite search engine) or feel free to ask more questions.
You can also use the READ (read the whole line into a string) and WRITE (append a string to the line) functions from std.textio package.