The core store of Atlas is provided with a form of indirect addressing which enables the supervisor to re-allocate areas of store and to alter their physical addresses, and which is also used to implement automatic drum transfers. With each page, or 512 word block, of core store there is associated a page address register which contains the most significant address bits of the block of information contained in the page. Every time access is required to a word of information in the core store, the page containing the word is located by hardware. This tests for equivalence between the requested block address, or most significant address bits, and the contents of each of the page address registers in parallel. Failure to find equivalence results in a non-equivalence interruption. The page address registers are themselves addressable in the V-store and can thus be set appropriately by the supervisor whenever information is transferred to or from the core store.
One of the most important consequences of this arrangement is that it enables the supervisor to implement automatic drum transfers. The address in an instruction refers to the combined core and drum store of the computer, and the supervisor records in subsidiary store the location of each block of information; only one copy of each is kept, and the location is either a page of core store or a sector of the drum store. At any moment, only some of the blocks comprising a particular program may be in the core store and if only these blocks are required, the program can run at full speed. When a block is called for which is not in the core store, a non equivalence interruption occurs, and the supervisor is entered to take appropriate action: a) the new block is transferred from a sector of the drum to a page of the core store, or b) a new block is allocated to the program, or c) the program is monitored if its block allocation has been exceeded, or d) the program is monitored if the new block is in the private store or a locked-out area of core store.
While a program is being transferred from drums to core store, the program that was interrupted is halted by the supervisor.
The block directory in subsidiary store contains one entry for each block in the combined core and drum store. It is divided into areas for each object program which is in the store; a separate program directory defines the area of the block directory occupied by each program. The size of this area, or the number of blocks used by a program, is specified before the program is obeyed in the job description (see Section 6). The entry for block n contains the block number n together with the number of the page or sector occupied by the block, and, if possible, is made in the nth position in the area; otherwise the area is filled working backwards from the end. In this way, blocks used by different object program are always kept distinct, regardless of the addresses that are used in each program. A program addresses the combined one-level store and the supervisor transfers blocks of information between the core and drum store as required; the physical location of each block of information is not specified by the program, but is controlled by the supervisor.
There are occasions when an object program must be prevented from obtaining access to a page of the core store, such as one involved in a drum or tape transfer. To ensure complete protection of such pages, an additional bit, known as a lock out bit, is provided with each page address register. This prevents access to that page by the central computer, except when on interrupt control, and any reference to the page causes a non-equivalence interruption. By setting and resetting the lock out bits, the supervisor has complete control over the use of core store; it can allow independent object programs to share the core store, it can reserve pages for peripheral transfers and can itself use parts of the core store occasionally for routines or working space, without any risk of interference. This is done by arranging that, whenever control is returned to an object program, pages that are not available to it are locked out.
A block of information forming part of an object program may also be locked out from use by that program because an operation on that information, controlled by the supervisor, is not complete. A drum, magnetic tape, or peripheral equipment transfer involving this block may have been requested. The reason for the lock out of such a block is recorded in the block directory, and if the block is in the core store, the lock out digit is also set. If reference is made to such a block by the object program, a non-equivalence interruption occurs and a supervisor extracode routine halts the program. This S.E.R. is restarted by the co-ordinator routine when the block becomes unlocked, and the object program is re-entered when the block is available in core store.
The drum transfer routine is a group of S.E.R.'s which are concerned with organising drum transfers, and updating page address registers and the block directory. Once initiated, the transfer of a complete block to or from the drum proceeds under hardware control; the drum transfer routine initiates the transfer and identifies the required drum sector by setting appropriate bits in the V-store. It also identifies the core store page involved by setting a particular dummy block address, recognized by the drum control hardware, in the page address register; at the same time, this page is locked out to prevent interference from object programs whilst the transfer is in progress.
On completion of a transfer, an interruption occurs which enters the drum transfer routine. The routine can also be entered from the non-equivalence interrupt routine, which detects the number of the block requested but not found in the page address registers. Finally, the drum transfer routine can be activated by other parts of the supervisor which require drum transfers, and by extracode instructions which provide a means whereby object programs can if they wish exert some control over the movement of blocks to and from the drum store. A queue of requests for drum transfers, which can hold up to 32 requests, is stored in the subsidiary store; when the drum transfer routine is entered on completion of a transfer, the next transfer in the queue is initiated.
Whenever the supervisor wishes to enter another request for a drum transfer, three possible situations arise. Firstly, the queue is empty and the drum transfer can be started immediately. Secondly, the queue is already partly filled and the request is entered in the next position in the queue. Thirdly, the queue is full. In this case the routine making the request is halted by the co-ordinator routine, and is resumed when the queue can receive another entry. In the first two cases the supervisor routine is concluded when the request reaches the queue.
A non-equivalence interruption, which implies a drum transfer is required, is dealt with as follows. The core store is arranged to always hold an empty page with no useful information in it, and when required, a transfer of a block of information from the drum to this empty page is initiated. Whilst this drum transfer is proceeding, preparation is made to write up the contents of another page of core store to the drum to maintain an empty page. The choice of this page is the task of the learning program which keeps details of the use made of blocks of information. This learning program will be described in detail elsewhere; it predicts the page which will not be required for the largest time, and is arranged with a feed-back so that if it writes up a block which is almost immediately required again, it only does this once. The number of the chosen page is recorded in the subsidiary store and the drum queue entry is converted to a request to write this page to the drum. This supervisor routine is now concluded and returns control to the co-ordinator routine.
When the drum transfer is completed, the drum transfer routine is again entered. This updates the block directory and page address register, makes the object program free to proceed and initiates the next drum request, which is to write the chosen page to the drum. This routine is now concluded and the co-ordinator is re-entered. The supervisor is finally entered when the write to drum transfer is complete. The block directory is updated, a note is made of the empty page, and the next drum request is initiated.
Some routines of the supervisor are obeyed in the main store and these and others use working space in the main store. Since the supervisor is entered without a complete program change, special care must be taken to keep these blocks of store distinct and protected from interference. The active supervisor blocks of main store are recorded in the area for program 0 in the block directory. There are also some blocks of the supervisor program which are stored permanently on the drum; when one of these permanent blocks is required, it is duplicated to form an active block of the supervisor or, as in the case of monitor routines, to become part of an object program.
Of the possible 2048 block numbers, 256 are reserved block numbers which are used exclusively by the supervisor and are not available to object program; object program are restricted to using the remaining non-reserved block numbers. Blocks with reserved block numbers may be used in the core store at any time by the supervisor, which can ignore the lock-out bits of these blocks. The supervisor also uses some blocks having non-reserved block numbers to keep a record of sequence of blocks of information such as input and output streams. When a non-reserved supervisor block is called to the core store, the page address register is not set, since there may be a block of an object program which has the same block number already in the core store. Instead, the page address register is set to a fixed reserved block number while it is in use, and is cleared and locked out before control passes to another routine, or before access to another such block is required.
Not all the reserved block numbers are available to the supervisor for general use, since certain block numbers are temporarily used when drum, tape and peripheral transfers are proceeding. These block numbers do not appear in the block directory. For example, when a magnetic tape transfer is taking place, the page of core store is temporarily given a block number which is recognized by the hardware associated with that tape channel. When the transfer is complete, the appropriate block number is restored. During a peripheral transfer, and also on other occasions, it is necessary that a block should be retained in the core store and should not be transferred to the drum. The relevant page of core store is locked down by setting a digit in the subsidiary store; the learning program never selects for transfer to the drum a page for which this lock-down digit is set.
