As mentioned in the Introduction, a large number and variety of peripheral equipments may be attached to Atlas. However, the amount of electronics associated with each equipment is kept to a minimum, and use is made of the high computing speed and interruption facilities of Atlas to provide control of these equipments and large scale buffering.
Thus the paper tape readers, which operate at 300 or 1000 characters per second, set an interrupt flip-flop whenever a new character appears. (Characters may be either 5, 7 or 8 bits depending on which of three widths of tape is being read.) Similarly, the paper tape punches, and the teleprinters which print information for the computer operators, cause an interruption whenever they are ready to receive a new character; these equipments operate at 110 and 10 characters per second respectively.
The card readers read 600 cards per minute, column by column, and interrupt the computer for every column. The card punches, at 100 cards per minute, punch by rows and interrupt for each row.
The Anelex line-printers have a print barrel, containing 64 different characters, rotating at 1000 revolutions per minute; there are 120 print positions spaced along the print barrel, a complete line being printed at a time. The printer contains a one-line buffer, and causes an interruption when it is ready to receive a new line of information. A songle line feed can be completed in a quarter of a revolution, and if only 48 consecutive characters on the print barrel are being used it is possible to print 1000 lines per minute. If all 64 characters are used, four lines may be printed every five revolutions of the print barrel, i.e. 800 lines per minute.
All the information received from, or sent to, these peripheral equipments is communicated via particular digit positions in the V-store. For example, there are eight such bits for each tape reader, and 80 for each card punch, together with a few more bits for control signals.
The majority of interruptions can be dealt with simply by the interrupt routine for the particular type of equipment. Thus the paper tape reader interrupt routine normally detects terminating characters and makes a parity check if required and, if all is well, stores the character to await code conversion by the Peripheral Extracode Routine (or P. E. R.).
P.E.R.'s perform the conversion of all input information into Atlas internal code, following input interrupts; and the conversion from Atlas code to the code of the requested output peripheral, before the output interrupt occurs. These conversions are suppressed during binary input and output.
The card routines are complicated by the check reading stations: punching is checked one card cycle afterwards, and reading is checked one column later. The interrupt routines apply these checks, and in the event of a failure, a monitor S.E.R. is entered.
Whenever equipment needs attention it is disengaged from the computer. In this state, which is indicated by a light on the equipment and a corresponding bit in the V-store, it automatically stops and cannot be started by the computer.
The operator may engage or disengage an equipment by means of two buttons so labelled. The equipment may also be disengaged by the computer by writing to the appropriate V-store bit, but the computer cannot engage it.
The engage and disengage buttons do not themselves cause interruptions of the central computer. Instead, the engaged bits in the V-store are examined every second (this routine is activated by the clock interruption) and any change activates the appropriate S.E.R. Disengaging a device does not inhibit its interruptions, so that if the operator disengages a card machine in mid-cycle to replenish the magazine or to empty the stacker, the cycle is completed correctly.
There are also other special controls for particular equipments, e.g. a run-out key on teletypes, and tape width selector switches on punched tape readers.
Most devices have detectors that indicate when cards or paper are exhausted or running low. These correspond to bits in the V-store that are read by the appropriate S.E.R. The paper tape readers however have no such detector, and the unlikely event of a punched tape passing completely through a reader (due to the absence of terminating characters) appears to the computer merely as a failure to encounter a further character within the normal time interval. This conditions is detected by the one-second routine.
In general, input information is converted to a standard 6-bit internal character code by the S.E.R. concerned, and placed in the store 8 characters to a word. (An exception to this occurs in the case of card readers when they are reading cards not punched in a standard code, in which case the 12 bits from one column are simply copied into the store and occupy two character positions. A similar case is punched tape, when this is used to convey information bits without a parity check. 5-, 7-, and 8-track tape can be read as binary input in all cases 12 bits are transferred for each character, only the bottom 5, 7 or 8 bits being significant. Such information is distinguished by warning characterds, both on the input medium and in the store.)
A certain amount of supervisor working space in the core store is set aside to receive this information from the interrupt routines, and is subdivided between the various input peripherals. The amount of this space depends on the number and type of peripherals attached. (At Manchester it is 1 block, at London 2 blocks and at Chilton 3 blocks; one block = 512 words.) These blocks will be locked down pages of the core store.
As each input equipment fills its share of this area, the information is copied by an S.E.R. into one block devoted exclusively to that equipment. These copying operations are sufficiently rare that the latter block need not remain in core store in the meantime; in fact it is subject to the same treatment as object programs by the drum transfer routine, and may well be put onto a drum and brought back again for the next copying operation. Thus only one, two or three pages of core store are used full time during input operations, but nevertheless each input stream finds its way into a separate set of blocks in the store.
The area that is shared between input peripherals is subdivided in such a way as to minimise the number of occasions on which information must be copied to other blocks; it turns out that the space for each equipment needs to be roughly proportional to the square root of its information rate.
Similarly, information intended for output is placed in the common input/output area, subdivided for the various output devices, and is taken from there by the interrupt routines as required. As soon as the information for a particular device is exhausted, a P.E.R. is activated to copy fresh information into the common area. The P.E.R. converts the internal code character into the code used by the device, and in the case of the card punch forms an image of the card, as the information is required in rows of bits.
As for input, the space allocated to each output peripheral is roughly in proportion to the square root of its information rate.
