MTEST4 is a memory test program originally written by Martin Eberhard to find a difficult memory issue in his MITS Altair 680 -- you can read about that here, on Herb Johnson's site.

MTEST4 tests a range of memory specified by the user on the console. It can relocate itself to anywhere in memory outside of page 0 (0x0000 to 0x00FF). It is capable of testing anywhere in memory outside of where MTEST4 itself is currently located. Do note that MTEST4 cannot currently test all of page 0 memory on the Altair 680! MTEST4 currently uses some of the monitor's I/O routines, which use variables located in page 0.

From Martin Eberhard's original notes:

The memory test algorithm is designed to ferret out a variety of memory problems. It is based on a test pattern that contains two walking bit patterns - one with the bit high, and the other with the bit low. It also contains various high-frequency patterns like 55 and AA. These pattern elements are scrambled to maximize the number of times each memory bit changes value during the test. The entire pattern is deliberately 29 bytes long -- a prime number. The pattern is written repeatedly through the test memory range, then read back and checked. Then the pattern is incremented for another pass through memory. This is repeated until every memory location in the test range has been tried with each byte in the test pattern. This should catch all address-line shorts, data line shorts, coupling between nearby cells, stuck bits, and even catch some longer-term memory loss faults.

In addition to Martin's original use case, MTEST4 was used to test and debug our Altair 680 Universal RAM board. Its use exposed a few edge cases we likely wouldn't have found without it! Additionally, it is very fast for how thorough it is.

Currently, MTEST4 uses some monitor routines in the MITS Altair 680 ROM monitor, and requires this monitor to be present at the usual location (0xFF00 to 0xFFFF).

The relocatable nature of MTEST4 makes the code bigger and less performant than it would otherwise be. Some of the subroutines have odd ordering to make them reachable by BSR instructions, thus allowing relocation. The source is commented to reflect this.

The mover program, which relocates MTEST4 from its initial load address to the user's desired run address, does check for valid data after moving. If there's an error in this process (bad/missing memory at the specified run address), it will abort to the ROM monitor.

This project requires the Glitch Works modified version of the A68 assembler to be compiled and available on your $PATH. A68 can be found in our GitHub repository.

Once A68 is available on your path, just type make to assemble. Default output format is Motorola S-Record, which is loadable by many ROM monitors found on Motorola 6800 systems.

Load the file MTEST4.obj into an Altair 680b using the PROM Monitor's 'L' command. Then jump to it at 0100 by typing 'J 0100'. Follow the directions - MTEST will first ask you where to put the code - pick an address outside where you want to test, and MTEST will relocate itself there, assuming there is actually memory at the address you specified. (If not, MTEST will abort to the PROM Monitor.)

Next, tell MTEST the address range of the memory to test. MTEST will run through 30 passes over the range. Each pass takes about 10 seconds for each 16Kbytes of tested memory. MTEST will print the pass number at the end of each pass. (Note that pass 0 only fills memory - no testing occurs.) Every time MTEST finds an error, it will report the address, what it wrote, and what it read.

The following enumerates revisions to the monitor before tracking in this particular Git repository:

• 2011-10-23 M. Eberhard Created
• 2011-10-27 M. Eberhard Don't use page 0 memory, so we can test it
• 2011-10-27 M. Eberhard Make code relocatable and relocating
• 2011-11-03 M. Eberhard comp. tsts, and check memory immediately
• 2011-11-03 M. Eberhard combine fill and test passes for speed
• 2023-12-18 J. Chapman Ported to modified WCC3 A68 cross-assembler
• 2023-12-24 J. Chapman General cleanup, no program changes

Original work is Copyright (c) 2011 Martin Eberhard. Modifications are Copyright (c) 2023 Glitch Works, LLC and are released under the GNU GPL v3.

MTEST4 was ported to assemble with a modified version of Warren C. Colley III's A68 cross-assembler (Glitch Works modified version, Herb Johnson's version). MFE's original version was written to assemble with PseudoCode's A68.com, a DOS-based cross assembler for the MC6800. Porting to A68 removed the workarounds for bugs in the PseudoCode assembler -- e.g. not correctly using direct addressing mode for some instructions.