Hmmm...this brings back some memories. From my recollection, this statement accepted a string argument. The character string supplied was coded in a special pseudo-hexadecimal: 0="0", 1="1", 2="2", 3="3", 4="4", 5="5", 6="6", 7="7" 8="8", 9="9", a=":", b=";" c="<", d="=", e=">", f="?" For example, to represent the hex sequence 6e7f391d, the string would be "6>7?391=" Each two characters represent a byte. Now, using this method, you could 'code' 6800 machine code instructions. The EXEC statement would store the bytes supplied in the string argument as a linear sequence of machine code, and cause a jump to the segment of code. I don't recall if it was JMP'd to, or JSR'd to...and don't remember any of the details of how to return from a segment of code executed with the EXEC statement. Obviously, sending random stuff to an EXEC statement could wedge the machine up. I remember that EXEC was used frequently by folks who were experts with the machine (and had magic lists of the internal runtime calls) to make game programs and stuff in machine code that would run much faster than the interpreted BASIC. It also allowed 'extensions' to BASIC that you could use to do things like PEEK and POKE, etc. There were conventions for passing variables in and out of EXEC'd code, so BASIC could be used for 'high level' functions, and the fast EXEC'd machine code could be used for time-critical or other functions that were too slow, or too tedious (IE: bit manipulation) to do in BASIC. Hope this helps. It's all from foggy memories..all the fun with 4051's that I had happened sometime around 1978...21 years ago. I'm amazed that I can remember ANY of it, given that many times I can't remember where I put my car keys :-)