Floating device address space

The floating device address space is a block of address space in the I/O page of the UNIBUS (and later the QBUS) used to hold the control registers of peripherals. Originally, each specific peripheral was assigned a fixed address for its registers; 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 address space (on the UNIBUS, from 0760010 to 0763776) 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. There is an empty minimum-sized group for each type of which there are no devices of that type present, and a gap between each group of devices (to mark the end of each group). (Each actual group must start on an address of the required modulus; e.g. if there is one DJ11 and one DH11, the DJ11 would be at 0760010, the 'end of DJ11s gap' would be at 0760020, the DH11 could not be at 0760030, but would have to be at 0760040.)

Floating devices, in assignment order:

Rank	Device	Size (modulus)	Comment
1	DJ11	010
2	DH11	020
3	DQ11	010
4	DU11	010
5	DUP11	010
6	LK11	010
7A	DMC11	010
7B	DMR11	010
8	DZ11	010
9	KMC11	010
10	LPP11	010
11	VMV21	010
12	VMV31	020
13	DWR70	010
14	RL11	010	Only extra
15	LPA11	020	Only extra
16	KW11-C	010
17	Reserved	010
18	RX11	010	Only extra
19	DR11-W	010
20	DR11-B	010	After second
21	DMP11	010
22	DPV11	010
23	ISB11	010
24	DMV11	020
25	DEUNA	010
26	UDA50	04	Only extra
27	DMF32	040
28	KMS11	20
29	VS100	20
30	TU81	04	Only extra

See also

• Floating device vectors

Further reading

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

External links

• peripherals handbook (1981-82) - Appendix D, 'Floating CSR Address Devices' (pg. 494 of the PDF]