MemMXtest is a computer program for "IBM-compatible" PCs which checks the computer's major memory for finding out any faults and defects in it. This kind of program are also made with an intention specially at testing the SDRAM memory modules (`DIMMs'), and also for making use of the MMX directions to read/write all the 64 data bits offered by these modules at once. MemMXtest includes with several well-known `march' tests, and many pseudo-random tests. The important parts of each test use manually optimized machine code for the maximal velocity of it. This program is a "stand-alone", and it accumulates on Linux.