BM792 ROM

The BM792 ROM card was the first UNIBUS ROM card for the PDP-11 series of computers; it contained only 32 words of memory. It was a quad format card, and used an SPC slot. Most versions were used for bootstrapping the machine, from disk, paper and magnetic tape, etc, but it could be used for any purpose

It used individual diodes in an array to represent individual bits; if the diode was present, the bit contained a '1'; if the diode was not present, a '0'. Jumpers allowed the board to occupy any 32-word block from 0773000 to 0773700, so longer programs could be accomodated with multiple cards.

The board came in two main variants: the M792 came fully populated with diodes, and the user removed diodes (either by de-soldering them, or clipping their leads) to produce the desired program; other variants, designated M792-Yx (where 'x' is a capital letter, starting with 'A') came pre-programmed from DEC to perform various functions.

Variants

• BM792-YA - Serial line, high-speed paper tape reader
• BM792-YB - TC11 DECtape and disks (RC11, RF11, RK11, RP11)
• BM792-YC - CR11 punched card reader
• MR11-DB - A two board set, consisting of the BM792-YD and -YE, it supported the same devices as the -YB, with the addition of the TM11 magnetic tape
• BM792-YF - DECtape and RF11 and RK11 disks
• BM792-YH - TA11 casette magnetic tape
• BM792-YK - Special version for the VT20, using its second serial line

Note that there are two variants of the -YA (see the link below).

See also

• UNIBUS boot ROMs

External links

• Dis-assembled and commented dump of M792-YA boot ROMs
• Dis-assembled and commented dump of M792-YK boot ROMs

Further reading

• DEC-11-HBMAA-E-D, BM792 read-only-memory and MR11-DB bootstrap loader