The Memory Loader window can load data from disk files into memory addresses. It is a special Memory window. It’s intended use is to load DEC ROM images. Several file formats are recognized.



To load data, use the controls in an up-to-down sequence:

  1. Choose a file format.
    (If a format is missing, feel free to contact me)
  2. Depending on the file format, select one ore more disk files.
  3. If the file format does not define a position for the data, choose a start address. Plain binary files require this.
  4. Press “Load”, the data appears in the grid.
  5. Press “Deposit all”. Data are transfered to PDP-11 memory ... this may take VERY long!
  6. You can verify the correctness of loaded program code with the Disassembly window.

You can edit the loaded data in the grid. Since all memory addresses are regarded to be “new”, they are highlighted until deposited. All addresses are physical 18 bit or 22 bit addresses.

In the example above, data is loaded from a binary ROM image. Since PDP-11 is a 16 bit machine, often two 8 bit ROMs are used in parallel. So all data bits 7:0 are saved in a “low-byte” file, and bits 15:8 are saved in a “high-byte” file.

Verifying the download

After "Deposit all", the memory is filled with data from the file. You can verify the memory content against the file by pressing the "Verify" button. This causes all memory cell to be read again.

If a memory cell differs from the file, the cell is painted yellow in the memory grid. So you have to scroll through the whole grid to check this.

Verifying memory is useful in a couple of situations:

  • Immediately after download you can test the function of the console driver. or the PDP-11 cpu.
  • you can use it as a memory test: if memory cells are defective, the verify will show differences.
  • if you downloaded a program which executed not well, you can check wether the crash damaged the code.

Turbo download in SimH

If you are connected to SimH, there’s an option to speed up the download: Right click into the grid and execute “Export as SimH script”. Save the memory content as “ \.sim”. Then goto the Terminal window and let SimH load the file by typing the command
“do \.sim”.
This is most easy if SimH is running on “localhost”.

Running paper tape images

In PDP11GUI v1.36, the "Absolute Paper Tape" format was added.

This allows you to read in DEC paper tape images from bitsavers. Encoded into these images is sometimes an entry address, where code execution should begin. This is also displayed in the memory load window. "00001" means: no entry address defined, then search in docs for it. The execution address is automatically copied to the Execution Control Window.

To execute a paper tape image:

  1. load it with the memory loader
  2. "Deposit" it into memory
  3. Switch to the Execution Control Window, verify the entry address there as start  address and click "Start"

You may try it with the attached image ... a 1970 BASIC interpreter. More about it here and here. (Thanks to Mattis Lind from, for support) -- PDP-11 BASIC on paper tape