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Chisel Memories

Chick Markley edited this page Jan 11, 2019 · 6 revisions

Loading Memories in simulation

Chisel now supports an experimental method for annotating memories to be loaded from a text file containing hex or binary numbers. When using verilog simulation it uses the $readmemh or $readmemb verilog extension. The treadle simulator can also load memories using the same annotation.

How to annotate

Assuming you have a memory in a Module

  val memory = Mem(memoryDepth, memoryType)

At the top of your file just add the import

import chisel3.util.experimental.loadMemoryFromFile

Now just add the memory annotation using

  loadMemoryFromFile(memory, "/workspace/workdir/mem1.txt")

The default is to use $readmemh (which assumes all numbers in the file are in ascii hex), bu to use ascii binary there is an optional third argument. You will need to add an additional import.

import chisel3.util.experimental.MemoryLoadFileType
  loadMemoryFromFile(memory, "/workspace/workdir/mem1.txt", MemoryLoadFileType.Binary)

See: /freechipsproject/chisel-testers/src/test/scala/examples/ComplexMemoryLoadingSpec.scala and /Volumes/UCB-BAR/chisel-testers/src/test/scala/examples/LoadMemoryFromFileSpec.scala for working examples.

Notes on files

There is no simple answer to where to put this file. It's probably best to create a resource directory somewhere and reference that through a full path. Because these files may be large, we did not want to copy them.

Don't forget there is no decimal option, so a 10 in an input file will be 16 decimal

Aggregate memories

Aggregate memories are supported but in bit of a clunky way. Since they will be split up into a memory per field, the following convention was adopted. When specifying the file for such a memory the file name should be regarded as a template. If the memory is a Bundle e.g.

Bundle {
 val a = UInt(16.W)
 val b = UInt(32.W)
 val c = Bool()

The memory will be split into memory_a, memory_b, and memory_c. Similarly if a load file is specified as "memory-load.txt" the simulation will expect that there will be three files, "memory-load_a.txt", "memory-load_b.txt", "memory-load_c.txt"

Note: The use of _ and that the memory field name is added before any file suffix. The suffix is optional but if present is considered to be the text after the last . in the file name.

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