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Getting a runtime error_0067 Value -1 out of range (0 to 7). Buf_ptr is set as a natural number but still goes negative because it is not overflowing back to 7. This can easily be fixed with an if statement but is there a setting in ActiveHdl that I need to enable to allow naturals to overflow?

.....
signal buf_ptr      : natural range 0 to 7
....
....
....
dma_wrt_Data(7 downto 0) <= incoming_data(buf_ptr);
bytes_available          <= bytes_available - 1;
buf buf_ptr              <= buf_ptr - 1;
packet_byte_count        <= packet_byte_count -1;
rcv_pkt_bct              <= rcv_pkt_bct - 1;

Scoured the web to see if anyone else had this problem, couldn't find any :(

Mikef
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jo132123sda
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  • While Ada95 introduced modular integers which would have the behavior you elicit with a type declaration containing the reserved word **mod**, VHDL was originally based on Ada83 and hasn't tracked. Modular integers also allow bitwise operations revealing the underling implementation type holding an integer type, while VHDL retains the Ada83 'pure' mathematical definition and requires you to avoid overflow/underflow in your design description for simulation. (Integer types are mapped to a binary type by synthesis. Such mapping wasn't always available.) – user16145658 Jul 20 '23 at 21:27

2 Answers2

2

Ranges do not do this. Mod does:

buf_ptr <= (buf_ptr - 1) mod 8;

Like @Tricky said, the purpose of a range is to allow checking of the bounds during an assignment.

Ada has modular types that may do this. These might be something interesting to have in VHDL, but someone would have to want to work on that aspect of the language. A starting point for this type of activity is: https://gitlab.com/IEEE-P1076/VHDL-Issues/-/issues

Jim Lewis
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VHDL is a strongly typed language. Hence integer types have no concept of over/underflow. When an object is assigned a value out of range it is a run-time error.

If you want explicit over/underflow, you can do this with the unsigned/signed types from ieee.numeric_std package.

Tricky
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