UNIBUS boot ROMs
From Computer History Wiki
DEC produced three generations of UNIBUS ROM boot cards.
The first was the BM792 ROM card, which used individual diodes in an array to represent individual bits; it contained only 32 words of memory. The second was the M9301 ROM card; it had a capacity of 512 words, using PROM chips. The third was the M9312 ROM card, which also had a capacity of 512 words.
In both of the latter two cards, the entire ROM contents were directly visible on the UNIBUS, not through a window.
