MIT UNIBUS XGP interface

The MIT UNIBUS XGP interface was a one-off UNIBUS peripheral which drove a Xerox Graphics Printer, the earliest raster printer (and thus the ancestor of the ubiquitous laser printer).

It used DMA to transfer rasters from the PDP-11's main memory to the XGP. A raster was 1700. bits long; in the PDP-11 memory, that took 108. words (1728. bits). (Some calculation took the raster length to be 1728. bits.) The raster buffers contained 3 additional words; these apparently were a 2-word header (below), and an "initial image mode word".

"Each [scan] line consists of a header, specifying how long it is, which line it is to be printed upon, and options .. These two words of header are followed by the line in the format required by the XGP interface.
The first PDP-11 word in each line is the number of PDP-11 WORDS .. used for this scan line, including itself and the rest of the header. [The buffer length appears to be in byte-swapped order.] ..
The second PDP-11 word is the scan line upon which this line is to be printed. [It is not clear if this in interpreted by the hardware as well, or just by the driver in the PDP-11.] It is allowable to have non-sequential lines. The unspecified lines will be blank. If the line numbers are not ordered, they will come out as close as possible together, and the internal line count will be reset. .. The top line on a page is numbered 1. ..
Paper cutting may be specified by setting the sign [high bit] of the second PDP-11 word. The rest of the [scan line] so marked will be ignored"

It is not yet clear if this last is interpreted by the hardware, or the driver in the PDP-11.

To print a scan line, load the Memory Address Register (below), set the FGO and FMOT bits in the Control Register (ditto); FDIE as well, to get an interrupt when done. The interface's interrupt vector is 0370.

The XGP at MIT was one of a number provided to various entities by Xerox in 1972; each had to produce its own hardware computer interfaces. MIT connected theirs to a PDP-11/20 (there is some confusion about exactly which model of PDP-11 it was, some think it was PDP-11/05). That was connected to the ITS KA10 MIT-AI via the Rubin 10-11 interface.

Device registers

Register	Abbreviation	Address
Control Register	XCR	772100
Memory Address Register	XMAR	772102
Status Register	XSR	772104
Cut Register	XCUT	772106

772100: Control Register (XCR)

FERR	Unused				FMOT	Unused		FDONE	FDIE	Unused			FCUTI	FCUT	FGO
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

The header of the driver (below) indicates that FCUTI in in this register, but this is believed to be an error. The code definitely indicates that this bit is in the XCUT register.

772102: Memory Address Register (XMAR)

BA15 <---> BA01															Unused
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

772104: Status Register (XSR)

FOS	FSYN	FOR	FNXM	FRDYC	FBCB	Unused		Status	FRDY	Status				Unused	FACT
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

772106: Control Register (XCUT)

Unused													FCUTI	Unused
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

Data encoding

The data pointed to by XMAR is encoded, but the format is not yet fully understood. The printer starts of in command or character mode.

A 0 byte escapes command mode. The next byte decides what mode to switch to: 0 enters run-length encoded mode. 1 stops the printer. 2 enters image mode. Anything else is an error.

In RLE mode, alternating bytes say how many pixels of white or black to draw. Two zeros in succession exits to command mode.

In image mode, bytes are imaged to pixels verbatim until the end of the page. Perhaps triggering an overscan error.

The MIT software uses the sequence 2 0 0 1 from command mode to make an empty line. It's not know what the 2 is here. Maybe the last two 0 1 is stop as per above.

External links

• SYSEN2; XGP > - code to run the interface
• INFO; XGP > - documentation on the spooler, and the input file format
• XGP - Xerox graphics printer - includes images of XGP's
• XCRIBL - A Hardcopy Scan Line Graphics System for Document Generation - a fair amount of detail on the CMU system
• Workshop on Hard Copy Line Graphics - details of the XGP installation at CMU