# Looking for help understanding a 'Reciproot' routine on the Manchester Mark I (1951)

I'm hoping to understand a routine written for the Manchester Mark I [NOTE: a comment from Raffzahn suggests this may be the Ferranti (wikipedia link) and not the Manchester Mark I] which computes the reciprocal square root (1 / √x). The Mark I had a small number of pre-written subroutines, 10 of which were in the manual for the Mark I when it was re-written in 1951 (Alan Turing's original manual was hard for some engineers to follow). A copy of the Reciproot routine is here

The routine entry contains both code and comments explaining the code. There are two tapes, Reciproot 1 and Reciproot 2. There is also an attached table which is for 'unstandardizing the results' (I am not sure what this means). Having code, comments and the necessary tables to follow the routine is very useful but as you can probably see the code is difficult to read, making it hard to check my intuition about what the comments describe. Here's what I mean (this is tape 1):

│/│VSTA│/│D@/J│E│
└─┤/C/@│E│VK/½├─┘
│GK/A│@│VK/A│
│/@PO│A│J@/J│
│GKPG│:│VK/½│
│GKPG│S│VK/A│
│/C/C│I│IS/J│
│E:IN│U│VK/½│
│/CTA│½│VK/A│
│/C/C│D│VK/½│
│J:/N│R│VK/A│
│VK/A│J│N@UJ│
│VKQO│N│N@UC│
│D:QG│F│VK/F│
│N@UC│C│VK/A│
│/C/N│K│BAT:│
│VK/A│T│US/C│
│VK/N│Z│GK/N│
│VK/N│L│GKTA│
│VK/A│W│GKQO│
│VKTK│H│OA/C│
│MKTA│Y│E:/O│
│MK/K│P│IS/C│
│A@/J│Q│VK/N│
│E@/½│O│:C/A│
│VK/A│B│:C/C│
│S@/J│G│E:/O│
│VK/½│"│/C/N│
│VK/A│M│/C/N│
│U@/J│X│IC/A│
│VK/½│V│QAPO│
│VK/A│£│KE/P│


I'm worried that if I try to do this by myself I'll spend forever on it and not make much progress in understanding things.

What I am looking for in an answer

I'm not a mathematician. I can follow, with some practice, code like this but it is hard work. Ideally I would like a straightforward explanation of what the code is doing, how it is computing the reciprocal square root, in a concrete fashion.

Some help in deciphering

Martin Campbell-Kelly wrote an excellent history of programming on the Mark I, Programming the Mark I: Early Programming Activity at the University of Manchester (public archive link. Among other things, it had a very idiosyncratic system for input code, based on the teletype machines available for input.

The Mark I instruction set is reproduced below from Campbell-Kelly's article (p. 137)

What this is for

I'm writing a history of the Quake III fast inverse square root (see 0x5f37642f.com) and this is the earliest extant program on a digital computer which computes a reciprocal square root. I hope to publish a paper where this plays a small role. An accepted answer will be cited in the paper (and a really good answer will get noted in the acknowledgments, hah).

I have tagged this as "floating-point" though I don't think this was a floating-point instruction. Retrocomputing doesn't have a tag for 'numerical' computing.

• Wow. nice question. Love it. Do you have the 'source' by chance as plain (and verified) ASCII? That might simplify decoding a lot. Also, Program as well as table seem to be not about the Manchester Mark 1 but the Ferranti Mark 1, as both included the extended instruction set - for example the A=A-D*S instruction, encoded as /½. Likewise the linked document mentions the use of B5 and B6, which are only present with the Ferranti Computer. Commented Feb 23, 2023 at 23:37
• I do not have the ASCII source. This could be the Ferranti. That might explain the discrepancy between the year Campbell-Kelly noted in his paper (1949) and the date on this document (1951). Thank you for that. Commented Feb 24, 2023 at 0:07
• @AdamHyland Is this related to the reciprocal square root implementation by Cicely Popplewell mentioned in the Wikipedia article about her? Turing mentioned reciprocal square root computation on the Mark I in a publication, apparently table based. A. M. Turing, "Some Calculations of the Riemann Zeta-Function," Proceedings of the London Mathematical Society, Vol. 3, No. 1, Jan. 1953, pp. 99-117: "By storing tables of log n and n ⁻¹ᐟ² within the machine ... The reciprocal square roots were given with error not exceeding 10⁻⁵" Commented Feb 24, 2023 at 6:54
• @AdamHyland Turing's 1953 publication I cited above relates to work performed in June 1950. According to Campbell-Kelly's 1980 publication in Annals Popplewell arrived at Manchester in October 1949 and the reciprocal square root routine found in the private papers of D. G. Prinz is dated November 1949. Which would indicate use of the original (prototype) Mark I, correct? I wonder whether the reciprocal square root computation used in the June 1950 computations is identical to the one dated November 1949. Commented Feb 24, 2023 at 7:20
• Re. canonical sources: there is a large archive of Ferranti materials in the basement of the Musuem of Science and Industry in Manchester. You need an appointment to visit. It might be worth a visit, should you happen to be in the area. Commented Feb 24, 2023 at 8:34

## Summary

The reciproot code calculates the reciprocal of the root using the Taylor-Series expansion around x = 1, that is,

1/sqrt(1+ε) = ∑_{k=0}  (-1)ᵏ (2k)! / (2ᵏk)!² εᵏ


or

  1           1     1⋅3      1⋅3⋅5
------  = 1 - - ε + --- ε² - ----- ε³ + ...
√(1+ε)        2     2⋅4      2⋅4⋅6


The Manchester/Ferranti only had integer computations, so everything is fixed point, which is tracked manually.

As the above formula only works close to 1, first the number is standardized (which we today call normalized), that is, shifted until it's as close to 1 as possible.

In a second step, to bring it closer to one, an approximation φ of the reciprocal square root is found from a table of 16 entries, which is then factored out.

The remaining value is of the form 1+ε for a sufficiently small ε, so in a third step, the reciprocal root of that value is calculated with 7 members of the above series, using the Horner's method.

Then φ is factored in again, and the resulting reciprocal root is calculated. With the reciprocal root it is easy to calculate the non-reciprocal root. Finally, two index registers are set up to keep track of the binary decimal point.

After calling the routine, one can choose where to put the binary decimal point. This is called understandardizing, as the value isn't standardized as described above. There is a table to assist with that based on assumptions on the range of the input, and where the decimal point should be, by choosing a value called S resp. S', and then using some additional code.

## Notation

The Manchester/Ferranti notation is a bit unusual from a modern perspective.

The contents of a memory address are written as [...].

The subscript + stands for "treat as unsigned number", while the subscript ± means "treat as signed number" (which is actually two's complement). The effect of the latter is to do a sign extension on load.

Alan Turing liked to use low-endian notations everywhere (see the introduction in [1]). I kept this for binary numbers (so the most significant bits are to the right), all other numbers are in normal notation.

Numbers are represented by a teletype code, so [/C] comes from / for binary 00000 and C for binary 01110.

New values (after the computation) are marked with a prime sign '. I have not figured out the superscript l or 1 in the link, potentially this was the result of a typist misreading the '.

## Programme Sheets

The "programme sheets" (or tapes?) are laid out in the same way as the binary digits on the tube.

20 bits form a horizontal (short) "line" (what we call a word today).

One the tube there are two columns of (short) lines with 20 bits each, in 32 rows. A long line (a double word) is formed from two short lines in consecutive rows. This is the data type for numbers.

On the sheet, the most-significant "address characters" (for the columns) are on the left resp. right side; the least-significant "address character" (for the rows) which is common to both columns is in the middle.

So this program contains 4 columns with address /, E, @ and A.

## Annotated Listing

A quickly made disassembly with some ad-hoc notation for the B-lines (index registers) follows.

//   VSTA  [VS]' = L, A' = 0        ; deposit link
E/   /C/@  std [/C]
@/   GK/A  [GK]' = M, M' = 0        ; M = position of MSB of [/C]
A/   /@PO  B6' = [/@]               ; 78
:/   GKPG  B6' = B6 - [GK]
S/   GKPG  B6' = B6 - [GK]          ; B6 = 78 - 2 * [GK]
I/   /C/C  D' = [/C]+
U/   E:IN  A' = A + D * [E:][B6]+   ; standard shift table??
½/   /CTA  [/C]' = L, A' = 0        ; [/C] now has MSB at bit 39
D/   /C/C  D' = [/C]+
R/   J:/N  A' = A + D * [J:]+       ; [J:] is probably 2^20 * 1/16
J/   VK/A  [VK]' = M, M' = 0        ; [VK] = 0..15 + [D:]
N/   VKQO  B7' = [VK]
F/   D:QG  B7' = B7 - [D:]
C/   N@UC  D' = [N@][B7]+           ; table with 16 overlapping entries of 20 bits
K/   /C/N  A' = A + D * [/C]+
T/   VK/A  [VK]' = M, M' = 0
Z/   VK/N  A' = A + D * [VK]+
L/   VK/N  A' = A + D * [VK]+
W/   VK/A  [VK]' = M, M' = 0
H/   VKTK  A' = 2[VK]±
Y/   MKTA  [MK]' = L, A' = 0        ; phi
P/   MK/K  D' = [MK]±               ; start Horner for [MK]
Q/   A@/J  M' = M + [A@]            ; 00000 00000 00000 00000  00000 00000 11100 11100
O/   E@/½  A' = A - D * [E@]+       ; 00000 00000 00000 00000  00000 00001 01101 01100
B/   VK/A  [VK]' = M, M' = 0
G/   S@/J  M' = M + [S@]            ; 00000 00000 00000 00000  00000 00000 00111 11100
"/   VK/½  A' = A - D * [VK]+
M/   VK/A  [VK]' = M, M' = 0
X/   U@/J  M' = M + [U@]            ; 00000 00000 00000 00000  00000 00000 00011 00010 = 0.2734375
V/   VK/½  A' = A - D * [VK]+
£/   VK/A  [VK]' = M, M' = 0

/E   D@/J  M' = M + [D@]            ; 00000 00000 00000 00000  00000 00000 00000 01010 = 0.3125
EE   VK/½  A' = A - D * [VK]+
@E   VK/A  [VK]' = M, M' = 0
AE   J@/J  M' = M + [J@]            ; 00000 00000 00000 00000  00000 00000 00000 00110 = 0.375
:E   VK/½  A' = A - D * [VK]+
SE   VK/A  [VK]' = M, M' = 0
IE   IS/J  M' = M + [IS]            ; out of the blue, use [IS]? Maybe 0.5?
UE   VK/½  A' = A - D * [VK]+
½E   VK/A  [VK]' = M, M' = 0
DE   VK/½  A' = A - D * [VK]+
RE   VK/A  [VK]' = M, M' = 0
JE   N@UJ  M' = M + [N@][B7]
NE   N@UC  D' = [N@][B7]+           ; table
FE   VK/F  A' = A + D * [VK]±
CE   VK/A  [VK]' = M, M' = 0
KE   BAT:  A' = 0                   ; also: address = BA
TE   US/C  D' = [US]+
ZE   GK/N  A' = A + D * [GK]+
LE   GKTA  [GK]' = L, A' = 0        ; sets ["K] = \alpha
WE   GKQO  B7' = [GK]
HE   OA/C  D' = [OA]+               ; 01011 11111 00110 01100  11110 01000 11010 01101
YE   E:/O  b-jump [E:]±             ; to QE (skip 1)
PE   IS/C  D' = [IS]+               ; 0.5?
QE   VK/N  A' = A + D * [VK]+
OE   :C/A  [:C]' = M, M' = 0        ; recip root
BE   :C/C  D' = [:C]+
GE   E:/O  b-jump [E:]±             ; to ME (skip 1)
"E   /C/N  A' = A + D * [/C]+
ME   /C/N  A' = A + D * [/C]+
XE   IC/A  [IC]' = M, M' = 0        ; root
VE   QAPO  B6' = [QA]               ; 40
£E   KE/P  jump [KE]+               ; to GA

/@   C@//  01110 01000 00000 00000  ; 78
E@   ////  00000 00000 00000 00000  ; a6 (taylor coefficient)
@@   /TPI  00000 00001 01101 01100
A@   ////  00000 00000 00000 00000  ; a5 (taylor coefficient)
:@   //UU  00000 00000 11100 11100
S@   ////  00000 00000 00000 00000  ; a4( taylor coefficient)
I@   //MU  00000 00000 00111 11100
U@   ////  00000 00000 00000 00000  ; a3 (taylor coefficient)
½@   //O½  00000 00000 00011 00010
D@   ////  00000 00000 00000 00000  ; a2 (taylor coefficient)
R@   ///R  00000 00000 00000 01010
J@   ////  00000 00000 00000 00000  ; a1 (taylor coefficient)
N@   ///N  00000 00000 00000 00110
F@   P/DP  01101 00000 10010 01101  ; 1   table to extract phi, 16 entries
C@   IJHY  01100 11010 00101 10101  ; 2
K@   ANEY  11000 00110 10000 10101  ; 3
T@   DMKH  10010 00111 11110 00101  ; 4
Z@   /P£W  00000 01101 11111 11001  ; 5
L@   DHTW  10010 00101 00001 11001  ; 6
W@   EW@W  10000 11001 01000 11001  ; 7
H@   BCYL  10011 01110 10101 01001  ; 8
Y@   L:DL  01001 00100 10010 01001  ; 9
P@   QZXZ  11101 10001 10111 10001  ; 10
Q@   AHLZ  11000 00101 01001 10001  ; 11
O@   BD½Z  10011 10010 00010 10001  ; 12
B@   XTVT  10111 00001 01111 00001  ; 13
G@   A½YT  11000 00010 10101 00001  ; 14
"@   ECNT  10000 01110 00110 00001  ; 15
M@   TE:T  00001 10000 00100 00001  ; 16
X@   ////  00000 00000 00000 00000
V@   ////  00000 00000 00000 00000
£@   ////  00000 00000 00000 00000

/A   VSTA  [VS]' = L, A' = 0.       ; deposit link
EA   KAQO  B7' = [KA]               ; 12
@A   /C/J  M' = M + [/C]
AA   /C/K  D' = [/C]±
:A   /C/F  A' = A + D * [/C]±
SA   :C/A  [:C]' = M, M' = 0
IA   :CTK  A' = 2[:C]±
UA   :CTA  [:C]' = L, A' = 0        ; [:C] := 2 * [/C] ^ 2
½A   E:QG  B7' = B7 - [E:]          ; loop: test B7
DA   E:/O  b-jump [E:]±             ;
RA   NS/P  jump [NS]+               ; return if not b-cond
JA   :C/C  D' = [:C]+
NA   :C/N  A' = A + D * [:C]+
FA   :C/A  [:C]' = M, M' = 0        ; [:C] = [:C]^2
CA   TA/P  jump [TA]+               ; to UA
KA   N///  00110 00000 00000 00000
TA   UA//  11100 11000 00000 00000
ZA   ////  00000 00000 00000 00000
LA   ////  00000 00000 00000 00000
WA   ////  00000 00000 00000 00000
HA   ////  00000 00000 00000 00000
YA   ////  00000 00000 00000 00000
PA   NE//  00110 10000 00000 00000  ; 44
QA   ½E//  00010 10000 00000 00000  ; 40
OA   G£NI  01011 11111 00110 01100
BA   K@JP  11110 01000 11010 01101
GA   "KPG  B6' = B6 - ["K]          ; start final part, B6 is 40
"A   "KPG  B6' = B6 - ["K]          ; B6 := 40 - 2*["K]
MA   VKPB  [VK]' = B6
XA   PAYO  B5' = [PA]
VA   VKYG  B5' = B5 - [VK]          ; B5 := 44 - B6 = 4 + 2*["K]
£A   NS/P  jump [NS]+               ; return


## Walkthrough

The code relies on constants in other locations, probably all of that was well-known at the time, but as it's not in the listing, I had to guess most of it.

Given the input value in [/C], which must be between 2^20 and 2^40, after completion, [/C] is now shifted, [:C] contains the reciprocal of the square root (shifted), and [IC] contains the square root itself.

The B-line registers B5 and B6 are also used and contain values related to the shift of the result.

In the first line //, the lower part of the accumulator is saved in VS. This is part of the "Schema A" to call a subroutine.

E/ standardizes [/C], and the following code until ½/ is used to shift [/C]. This seems to use a standard table of powers of two at //, since [E:] is probably zero.

From D/ to T/, a value in [J:] (probably 2^20 * 1/16) is multiplied to the shifted [/C], effectively dividing [/C] by 16. This is used as an index in a overlapping table of 20 bits with 16 entries. While 40 bits are loaded, the lower 20 bits seem to be ignored. The value in the table is multiplied to [/C], squared (Z/ to W/), doubled (H/), and then serves as the ε in the series.

P/ to RE apply Horner's method to evaluate the polynomial, with the coefficients in E@ to N@. One of the coefficients is [IS], another seemingly well-known constant which must be 0.5.

JE to CE adjusts the result with the factored-out value, using again the table.

KE to LE sets up ["K] with what the what is called α in the source text (the shift of the result).

WE to XE make the even-odd adjustments that are also mentioned in the source text, while storing the final computer reciprocal root in [:C], and the root itself in [IC].

VE to £E loads B6 with 40, and then continues at GA.

GA to "A set up B6 as described in the source text, while MA to VA do the same for B5.

Finally, £A returns to the called (apparently the return address is stored by convetion in [NS]).

Interstingly, /A to TA seem to contain a completely unrelated routine that calculates a large power of [/C] and return the result in [:C]. One can see the same "calling convetions" as in the other routine.

There is also a description of the routine in [1], page 80, under "standard routines", with the name RECROOT (and not RECIPROOT):

The routines for mathematical functions were mainly chosen for simplicity of programme and generality of application [rather] than for speed.

[...]

RECROOT.

This was a relatively slow method for the reciprocal square root. It depends on the case p = 2 of the recurrence relation

u_{n+1} = u_n (1 − 1/p * a * u_n ^ p)

for a^(-1/p). It was necessary for the argument to be within a relatively restricted range, and for u_0 to satisfy 0.5 < a * (u_0)^2 < 2. It was often found more convenient to use the logarithm and exponential, although much slower.

## Conclusions

• While the routine uses a lookup table and then does some calculations, there the similarity to the Quake fast inverse square root ends: Mathematically, the approaches are completely different (Taylor-series expansion vs. Newton iteration).
• They new that this method was slow, but used it for easy of programming (as a sort of "proof of concept"?)
• The trick to divide by a constant by multiplying with a different constant and then throwing away the lower part was also used back then
• Addressing of the mark 1 is a bit unusual from today's point of view, as it's 'bottom up'. While it's true that /C is binary 00000.01110, in addressing it means Line 0 (/) of Column 14 (C) which comes to address 448 - or the first line in 'unsystematic storage' - aka single variables. Commented Feb 24, 2023 at 13:27