Mercurial > repo

comparison interps/c-intercal/pit/lib/syslib.doc @ 996:859f9b4339e6

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
<Gregor> tar xf egobot.tar.xz
author HackBot
date Sun, 09 Dec 2012 19:30:08 +0000
parents
children
comparison
equal deleted inserted replaced
995:6883f5911eb7 996:859f9b4339e6
1
2 The file syslib_old.i is the version of the system library included
3 in the original manual. RTFM for descriptions of the routines. Only
4 minimal changes have been made in the current version, as follows:
5
6 1. Someone added
7 PLEASE KNOCK BEFORE ENTERING
8 as the first line, presumably to prevent programs from running off the
9 end and into the first library routine by mistake.
10
11 The other two changes are bug fixes. The bugs appear in all copies of
12 the library that I have, including the manual from Don Woods (via Mike
13 Ernst) with carriage controls in the first column. Unless a still more
14 ancient copy of the library can be found (like in the Dead Sea Scrolls,
15 perhaps?) we have to assume these bugs have been in there from the
16 beginning, or at least since the library was converted from EBCDIC to
17 ASCII and incorporated into the manual.
18
19 2. Jacob Mandelson found a bug in the 32-bit division routine where a
20 select (~) had been substituted for a mingle ($) by mistake. This is at
21 the end of line 229 of syslib_old.i. I have not checked or tested this
22 correction, but it seems reasonable since select at that position would
23 be pointless, but mingle is plausible.
24
25 3. I have found either two or three bugs in the 32-bit multiplication
26 routines, depending on how you count. These only affected the overflow
27 checking, but the effect was that every single call to (1540) or (1549)
28 would behave as if there were an overflow.
29
30 The first is straightforward. The statement
31 PLEASE DO :1 <- ":3~'#65280$#65280'"$":5~'#652
32 80$#65280'"
33 must be changed to
34 PLEASE DO :1 <- ":3~'#65280$#65280'"$":4~'#652
35 80$#65280'"
36 Not only is this the "right thing to do", but the position of this
37 statement just after :4 has been computed suggests that it was what
38 the original author intended.
39
40 The remaining problems, however, are not so clearly due to typos. The
41 32-bit multiplication is done by splitting each input into its low and
42 high 16-bit halves, computing partial products with the (1530) 16 to
43 32-bit multiplication routine, and then adding them together using
44 (1509), addition with overflow check. The statements to look at are
45 the ones where the overflow flags from successive steps are accumulated
46 in .5, as follows:
47
48 [a] DO .5 <- ':1~:1'~#1
49 ...
50
51 [b] PLEASE DO .5 <- '"':1~:1'~#1"$.5'~#3
52 ...
53
54 DO (1509) NEXT
55 [c] DO .5 <- !5$":4~#3"'~#15
56 ...
57
58 DO (1509) NEXT
59 [d] PLEASE DO .5 <- !5$":4~#3"'~#63
60 DO .5 <- '?"!5~.5'~#1"$#1'~#3
61
62 .5 gets a bit if the high-high product is nonzero (wasteful to even compute
63 this product, actually), and another if either of the high-low products
64 overflows (that's the statement with the previous bug, above). It then
65 gets combined with the overflow flags in :4 from the two addition calls.
66 Each new flag is mingled with the ones already in .5, so various bits of
67 .5 correspond to the different possible overflows. Finally, .5 is tested
68 to see if it is nonzero, and if so an overflow results.
69
70 Note the expressions ":4~#3". :4 is returned by (1509) as #2 if there is
71 an overflow, #1 if not. It either case a bit is set in .5. This is why
72 the routines signaled an overflow every time. Note also the ~#63 in line
73 [d]. At this point the bit representing existence of the high-high
74 product has shifted out to the 128-bit, so ~#63 will miss it.
75
76 I see three ways out, none of which are obviously the author's intent:
77
78 (1) At statement [d] the bit pattern is 101011XX, where 1's indicate
79 overflows and X's are set by (1509) without overflow. If we replace
80 ~#63 with ~#172 we test exactly the bits we want. This is the one
81 I've used since it is the minimal change.
82
83 (2) If we replace ":4~#3" with ":4~#2" we only have bits set on overflow,
84 which seems cleaner. The bit pattern is 10001011 so we still can't use
85 ~#63, we have to use ~#139 or ~#255 or something similar to get all the
86 bits. Actually, the final selects in lines [b], [c] and [d] are all
87 superfluous in this case since the bits discarded are all zero and the
88 selected quantity can be safely assigned to the 16-bit .5.
89
90 (3) If we replace ":4~#3" with ":4~#2" and reverse the mingles to look
91 like '":4~#2"$.5' then the final bit pattern is 10111 and ~#63 will work,
92 though it is not necessary.
93
94 There are other solutions, of course, but I don't see any simpler ones
95 than these. If you think ":4~#3" accomplishes nothing, so ":4~#2" must be
96 meant, note that ":4~#3" converts :4 to 16-bit. The original compiler
97 apparently required arguments to mingle to be 16-bit, whereas I think
98 the C-INTERCAL compiler only requires them to be less than #0$#256.
99 ":4$.5" would thus be technically illegal, even though it might work in
100 C-INTERCAL.
101
102 Louis Howell
103 May 25, 1996