Williams Tube Display

A Display for a Williams-Kilburn CRT Store

Storage Patter on C.R.T

Fig.6 A typical Storage Patter on C.R.T

Fig.6 shows the display from a Display Cathode Ray Tube (CRT) for a Williams-Kilburn CRT Store (from a Ferranti Mark 1). The bright spots represent 1s and the dim spots represent 0s, representing data in a binary format. Each spot corresponds to an area of electrostatic charge. The display shows 32 40-digit binary numbers with an extra 20-digit line at the top.

In the Baby the layout was 32 rows of 32-bit numbers.

On an actual Williams-Kilburn Tube, the layout of bits was regular and the display of 1s and 0s as phosphor illumination was incidental to the storage mechanism -- in fact it was obscured by the metal pick-up plate over the face of the CRT which sensed the charge. It was also necessary to enclose the CRT in a metal box to protect it from outside electrical interference (e.g. trams). So one or more Display CRTs were provided, each of which held a copy of the current values in a selected storage CRT, laid out in a convenient form for the onlooker. Until the paper tape punch was attached to the Manchester Mark 1, reading a Display CRT was the only form of output.

Ferranti Mark 1 Layout

To be precise, the 32 rows of 40 dots on a Display CRT for a Ferranti Mark 1 (see above) represented 64 addressable "line"s of 20 bits arranged in two columns of 32 20-bit lines side by side, the left column comprising addresses 0 to 31 and the right addresses 32 to 63. A line could contain a single instruction. A 40-bit number could be held at any address j and its successor j+1 (mod 64). The extra 20-bit line at the top of the left column was added to show the associated page on the magnetic drum store where relevant.

Note that although the array of bit positions on the corresponding Williams-Kilburn Tube was laid out in a regular array, for user convenience the rows of 20-bit lines are grouped in 4s and 8s, and the bits within a line in groups of 5. Five was chosen because input and output were provided by a five-hole paper tape reader and punch, so this permitted easy correlation between information in the computer store and any information on input/output 5-hole tape that corresponded to it. Note that the least significant digit is on the left hand side of a 5-bit group or a 20-bit line. (See also Programming on the Mark 1.)

Looking more closely, you can see that the integers at addresses (1,2) to (39,40) hold the powers of 2 from 0 to 19, and the integers at addresses (0,1) to (38,39) hold the powers of 2 from 20 to 39 -- a clever shared use of store! Integer (41,42) holds -1, and lines 44 to 63 hold instructions, with field 0-8 giving a 20-bit line address between 0 and 511, field 10-12 a B-line address 0 to 7, and field 14-19 the function code. These instructions contain the routine-changing sequence for Scheme A, and in fact the display is the PERM page (2) of Scheme A.

Manchester Mark 1 CRT

The Manchester Mark 1 used a CRT holding twice the number of bits.

Manchester Mark 1 Layout

The Manchester Mark 1 used a CRT holding twice the number of bits, with display given as two "pages" of 32 * 40 bit arrays side by side, as shown below (the right hand 32 * 40 array being all 0s). A word held a 40-bit number or two 20-bit instructions. The mechanism is described in detail in the M.Sc. thesis of D.B.G. Edwards.

Both the Williams-Kilburn Tube and the Display CRT were the same size as for the Baby and the Ferranti Mark 1 (i.e. based on a standard 6 inch CRT). However for reliability reasons the Ferranti Mark 1 stored only one page per Williams-Kilburn Tube, thus reverting to the smaller number of bits per Williams-Kilburn Tube (i.e. the order of 1,000 as in the Baby).