Floating device vectors

Floating device vectors are a block of interrupt vectors in low main memory on the UNIBUS (and later the QBUS). Originally, each specific peripheral was assigned a fixed address for its vector(s); once the number of different device types became large, circa 1975, this was no longer feasible (or scalable), and the 'floating' approach was adopted instead.

In this, a block of vectors (from 0300 to 0776) is allocated for 'floating' devices. Within that block, devices are assigned in a fixed order, with all the devices of any type in a contiguous group. If a device type which has only one vector (4 bytes) has an odd number of devices, and is followed by a device type with pairs (or more) of vectors, which generally are aligned on a 010 boundary, there will have to be an unused vector between them.

The floating devices, in assignment order, are:

Rank	Device	Size	Comment
1	DC11	010
1	TU58	010	No config for systems with DC11 + TU58
2	KL11	010	Extra units
2	DL11-A/DL11-B	010	Extra units
3	DP11	010
4	DM11-A	010
5	DN11	04
6	DM11-BB	04
7	DH11 modem control	04
8	DR11-A	010
9	DR11-C	010
10A	PA611	04	Reader
10B	PA611	04	Punch
11	LPD11	010
12	DT11	010
13	DX11	010
14	DL11-C/DL11-D/DL11-E	010
15	DJ11	010
16	DH11	010
17	GT40	020
17	LSV11	020
18	LPS11	030
19	DQ11	010
20	KW11-W	010
21	DU11	010
22	DUP11	010
23	DV11	014	Also modem control
24	LK11	010
25	DWUN	010
26A	DMC11	010
26B	DMR11	010
27	DZ11	010
28	KMC11	010
29	LPP11	010
30	VMV21	010
31	VMV31	020
32	VTV01	020
33	DWR70	020
34	RL11	04	Extra units
35	RX211	04	Missing from PDP-11 Architecture Handbook
36	TS11	04	Extra units
37	LPA11	010
38	IP11	04
39	KW11-C	010
40	RX11	04	Extra units
41	DR11-W	04
42	DR11-B	04	Extra units
43	DMP11	04
44	DPV11	04
45	ML11	02	MASSBUS device
46	ISB11	04
47	DMV11	04
48	DEUNA	02
49	UDA50	02	Extra units
50	DMF32	020
51	KMS11	06
52	PCL11-B	04
53	VS100	02
54	TU81	02	Extra units

It will be noticed that the types with an 'extra unit annotation are all mass storage devices, generally disks. This probably means that the 'base' unit is assigned a fixed vector, so that bootstraps do not have to contain the lengthy algorithm to deal with floating vectors in order to boot from them.

See also

• Floating device address space

Further reading

• PDP-11 Architecture Handbook (not online); 'Floating Vectors', in Appendix A

External links

• peripherals handbook (1981-82) - Appendix D, 'Floating Vectors' (pp. 492-493 of the PDF]