MemMXtest is a computer program for "IBM-compatible" PCs that checks the computer's main memory for faults and defects. (Note however, that no program can be guaranteed to detect all faults, due to their very nature.) This program is aimed specifically at testing SDRAM memory modules (DIMMs'), and uses MMX instructions to read/write all 64 databits provided by these modules at once. MemMXtest incorporates many well-known march' tests, as well as several pseudo-random tests. The vital parts of each test use manually optimized machine code for maximal speed. The program is "stand-alone" (i.e. doesn't require an OS to run), and compiles on Linux.