What happens to out of range memory addresses?

General discussions about using the Astrobe IDE to program the FPGA RISC5 cpu used in Project Oberon 2013
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What happens to out of range memory addresses?

Post by jmaggio14 » Tue Mar 16, 2021 11:35 pm

I see in RAM.v that 3 blocks of BRAM are instantiated for a total of ~48k words. Since the `adr` signal is 16bit and can theoretically address up to ~65k words, what happens if I pass in an address beyond 48K?

If I've done my math correctly, that means that any addresses 0xBFFF would be out of range? Is there a condition to handle this scenario? (We're looking to reserve sections on the address bus for our application, so I'm wondering if I can reserve these addresses without disrupting the RAM operation)

Code: Select all

  wire [31:0] do [0:2]; //3 32bit wires, one for each block
  // select RAM block using the 2 MSB in the address
  // does this mean that '11' is out of range? because there are only
  // 3 memory blocks?
  // '00' --> block0
  // '01' --> block1
  // '10' --> block2
  // what happens with '11' since there are only 3 blocks? 
  rdata = do[adr[15:14]];

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Re: What happens to out of range memory addresses?

Post by cfbsoftware » Wed Mar 17, 2021 12:37 am

Memory addressing limits are defined in Bootload.Mod. For the Artix-7 XC7A35T, using 192KB BRAM, MemLim is defined as 030000H. Read Section 8 Storage layout and management in the Project Oberon documentation (Astrobe menu > Help > Project Oberon) to see how the address range is partitioned and managed by Oberon.

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