The BM873 ROM card was a UNIBUS card for the PDP-11 series of computers; it was a quad format card, and contained 256 words of memory (optionally limited to 128 words), contained in four PROM chips (two, when the limitation option was selected).
The board appeared at addresses from 773000 to 773776; if the 128-word limitation is enabled, the top address is 773376. Several of the computers it was intended for use with (PDP-11/40, PDP-11/45) could be configured so that the host computer jumped to the code on the board on power-on.
In addition, an edge connector provided four external input lines, each of which (when pulled low) forced (via a simulated power-fail restart, and then forcing the first two read cyles after the power-up to read data specially provided by the board) the computer to jump to a particular address (configured by a group of 8 diodes, four groups in total - one for each input line) within the card's address space.
Jumpers W1 and W2 adapt the board to variations in fetching the new PS/PC after power-up - the bus cycles which the board interferes with, to force the new PC to go to a specific location within the board. Some machines, like the -11/40, fetch the PS before the PC; others, like -11/45, fetch the PC first.
Jumper W3, when installed, limits the card to 128 words.
Resistor R3 can be removed to disable the 'default address' feature. This is that on a system configured to fetch the new PS/PC from ROM on power-on - a capability which exists in the -11/40 and -11/45, when used with MOS memory - on a power-on, with no external line asserted, with this resistor in place, the board acted as if the first external input had been asserted.
In addition to an un-programmed variant, which the user could program as they wanted, the board came from DEC in several pre-programmed variants, designated BM792-Yx (where 'x' is a capital letter, starting with 'A'), set up to perform various functions. Many of the later variants were used to boot the PDP-11/40 used as the front-end processor in the KL10.
- BM873-YA - Boot from disks (RF11, RK11, RP11, RC11), TC11 DECtape, TM11 magnetic tape, TA11 casette magnetic tape, serial line, and high-speed paper tape reader
- BM873-YB - As above, with the addition of RS0x and RP0x disks and TU16 tapes; in addition, booting from units other than 0 (with the unit being specified in the console switch register)
- BM873-YC - As the -YA variant, with added support for the DU11 synchronous line interface
- BM873-YD - KL10 1080 front end; supported booting from, and dumping to, TC11, RP0x, and DTE20 10/11 interface
- BM873-YE - Unknown
- BM873-YF - 2040 console; same capabilities as -YD, except the RX11 replaced the TC11
- BM873-YG - DN87 communications; supported booting from a DL11 and the DTE20, and dumping to the latter
- BM873-YH - 10xx/20xx console; supported booting from TC11, RX11, RP0x, and possibly the DTE20 10/11 interface (the documentation is contradictory)
- BM873-YJ - 10xx/20xx console/communications; supported booting from TC11, RX11, RP0x, DL11 and the DTE20, and dumping to the latter
- EK-BM873-TM-004, BM873 restart/loader
- KL873.MEM, KL10 BM873 Bootstrap ROMs