16 The RK Disk Driver

The RK disk storage system employs a removable disk cartridge containing a single disk, which is mounted inside a drive with moving read/write heads.

The device designated RK11-D consists of a disk controller together with a single drive. Additional drives, designated RK05, up to a total of seven, may be added to a single RK11-D.

A requirement for more than eight drives would require an additional controller with a different set of UNIBUS addresses. Also the code in the file “rk.c” would have to be modified to handle the case of two or more controllers. This case is most unlikely because requirements for large amounts of on-line disk storage will be more economically provided otherwise e.g. by the RP04 disk system.

$\begin{tabbing} Cartridge \= capacity: \= 1,228,800 words\\ \> (4800 512 byte records)\\ Surfaces/cartridge: \> \> 2\\ Tracks/surface: \> \> 200 (plus 3 spare)\\ Sectors/Track: \> \> 12\\ Words/Sector: \> \> 256\\ Recording density: \> \> 2040 bpi maximum\\ Rotation speed: \> \> 1500 rpm\\ Half revolution: \> \> 20 msecs\\ Track positioning:\\ \> 10 msecs (one track)\\ \> 50 msecs (average)\\ \> 85 msecs (worst case)\\ Interrupt Vector Address: 220\\ Priority Level: \> \> 5\\ \end{tabbing}$

Unibus Register Addresses
Drive Status	RKDS	777400
Error	RKER	777402
Control Status	RKCS	777404
Word Count	RKWC	777406
Current bus address	RKBA	777410
Disk address	RKDA	777412
Data Buffer	RKDB	777416

Table 16.1 RK Vital Statistics

The average total access time is 70 milllseconds. With multi-drive subsystems, seeking by one drive may be overlapped with reading or writing by another drive. However this feature is not used by UNIX because of bugs which existed at one time in the hardware controller.

In initiating a data transfer, RKDA, RRBA and RKC are set, and then RKCS is set. Upon completion, status information is available in RKCS, RRER and RKDS. When an error occurs, UNIX simply calls “deverror” (2447) to display RKER and RKDS on the system console, without any attempt at analysis. An operation is repeated up to ten times before an error is reported by the device driver.

The register formats which are described fully in the “PDP11 Peripherals Handbook” are reflected in the program code at several points. The following summaries suffice to describe the features used by UNIX:

Control Status Register (RKCS)
bit	description
15	Set when any bit of RKER (the
	Error Register) is set;
7	Set when the control is no
	longer engaged in actively
	executing a function and is ready
	to accept a command;
6	When set, the control will issue
	an interrupt to vector address
	220 upon operation completion or
	error;
5–4	Memory Extension. The two most
	significant bits of the 13 bit
	physical bus address. (The other
	16 bits are recorded in RKBA.);
3–1	Function to be performed:
	CONTROL RESET: 000
	WRITE: 001
	READ: 010
	etc.,
0	Initiate the function designated
	by bits 1 to 3 when set. (write
	only);
Word Count Register (RKWC)

Contains the twos complement of the number of words to be transferred.

Disk Address Register (RKDA)
bit	description
15–13	Drive number (0 to 7)
12–5	Cylinder number (0 to 199)
4	Surface number (0,1)
3-0	Sector address (0 to 11)