It's configuration is as follows:
|Clock Speed||10 MHz||-|
This is one of the first "pair" of computers in the MicroVAX 3100 line of machines still markeded by Compaq. With referring to "pair" I want to indicate that the first "pizza box" models in the MicroVAX 3100 line were delivered in both a low profile and a higher profile version. The M20 has the higher profile and supports more internal storage than the M10 which has the low profile. In the MicroVAX 3100 (and also the VAXstation 3100 and VAXstation 4000) series Digital dropped QBus expansion and relied almost entirely on SCSI, with very limited expansion in other ways. Some special cards exists for the M20 to bring more serial ports, but otherwise there are not many choises.
In the M20 model memory can be expanded to a maximum of 32 MB by using special cards stacked on each other. By standard the machine has 4 MB of memory on the motherboard. The first memory expansion is attached directly to the motherboard, while the second expansion is attached via the first board. This chaining of boards is not even physically possible with all combinations! The board attached to the motherboard must be physically bigger than the one attached to it. To go to a full 32 MB a "big" 16 MB board has to sit first in the chain, and attached to it should be a smaller 12 MB board. This arrangement is similar for the MicroVAX 3100 M10/20, M10e/20e and VAXstation 3100 M30/40, M38/48, while later designs uses different kinds of SIMs although not always standard SIMs.