M9312 ROM

The M9312 ROM card was a UNIBUS card for the PDP-11 series of computers. It contained 512 words of ROM memory, all readable directly from the UNIBUS (i.e. not windowed). The M9312 also provided termination for the UNIBUS.

It was a dual format card, intended for use in the two top (AB) sections of a MUD slot, or in a UNIBUS in/out slot.

The board occupied addresses 773000-773776 (high ROM) and 765000-765776 (low ROM); a configuration jumper allowed the lower block to be disabled.

Configuration switches controlled which address the CPU jumped to on power on (a clever kludge, controlled by one configuration switch, allowed the board to force the CPU to read its power-on PC and PS from the ROM, at a location set by other configuration switches, thereby allowing auto-boot on power-on).

ROMs

The standard ROM contents provided the ability to bootstrap the machine, from disk, magnetic tape, etc, but the board could be used for any purpose. The DEC-supplied pre-programmed ROMs included a console emulator, and basic diagnostics.

It used five 4-bit wide PROMs to hold the data: one for the basic console and diagnostics, and the other four to hold the selected bootstraps (selected from a large set available from DEC),

Configuration switches

The board contains a single 10-switch DIP switch, S1, which together with a number of jumpers, controls the board's behaviour.

If S1-1 is set to 'off' (open), the new PS and PC loaded on power-on will point to the high ROM; if 'on' (closed), it will be in the low ROM.

If S1-2 is 'off', the system will power-up normally; if 'on', the CPU will obtain its new PC/PS pair from locations 173024 on the card, which contain a mixture of bits from the ROM (high bits) and the configuration switches (low bits), which can be used to force the CPU to start executing in the ROM on power-on.

Switches S1-3 through S1-10 contain bits 8 through 1 of the power-on PC (above).

Configuration jumpers

• W1-W5 - When inserted, termination is provided for the bus grant lines of the UNIBUS.
• W6 - In for machines with push-button boot capability
• W7 - Always in
• W8 - When inserted, the low bank of memory on the card (765000-765777) is disabled; when removed, it is enabled.
• W9-W10 - Out for use with a PDP-11/60; in for all others
• W11-W12 - In for use with a PDP-11/60; out for all others

Connector tabs

The board contains four connector tabs, which may be used with a pair of external switches, to control the board's behaviour.

Two, TP2 and TP3, are ground returns (for TP1 and TP4, respectively). TP1, when momemtarily grounded, forces the machine to execute a power-fail/restart sequence (by asserting UNIBUS line ACLO). TP4, when grounded, forces the machine to execute the on-board ROM on power on, no matter what the position of S1-2 (above).

See also

• UNIBUS boot ROMs

Reference

• EK-M9312-TM-003, M9312 bootstrap/terminator module technical manual