View source for CHECK/CHECK2/CHECK3

{{FormatInfo
|subcat=Error detection and correction
|released=July 1987
}}
'''CHECK/CHECK2/CHECK3''' was a Type-in Program Validator for (Mallard) [[BASIC]] program listings for the Amstrad PCW which were published in the UK's ''8000 Plus'' magazine (later renamed to ''PCW Plus'').

It generates one 4-hex-digit checksum for every line of program listing, case-insensitively and ignoring spaces. It operates on the ASCII (not tokenised) version of the listing.

== Version history ==
=== Original (buggy) version, 1987–1988 ===
The first version was published in issue 10 (July 1987) p60 of ''8000 Plus'' magazine (as a type-in BASIC listing). It was titled "BASIC Checker" and credited to Adrian Wilkins. All type-in listings in the magazine from this issue through issue 25 were accompanied by checksums generated by this program; the program itself was reprinted every few issues.

Unfortunately, this version was buggy -- while it would flag many mistakes, the checksum ignored every seventh character.

(John Elliott retrospectively refers to this version as 'check1'.)

=== CHECK2/3, 1988–1995 ===
A corrected version was published in issue 26 (November 1988) p64 under the name '''CHECK2.BAS'''; the correction was attributed to Peter Crane.

The checksums produced by this version were of course different from the first version. All type-in listings in the magazine from this issue through ''PCW Plus'' issue 106 (July 1995) were accompanied by these checksums.

(A third and last version of the checker, '''CHECK3.BAS''', was published in issue 43 (April 1990) pp60,63. There was no change to the checksum algorithm; the program just had more options for where to output the checksummed listing. This version was reprinted every few issues while checksummed listings were still being published.)

=== Unofficial versions ===
'''CHECK5''' (1994–5) by John Elliott extends the Mallard BASIC implementation to produce these checksums (compatible with CHECK2/3) directly, rather than having to save the program being keyed and feed it to a separate program. ("CHECK4" was an earlier attempt at the same thing by the same author.)  <!-- FIXME link to chek5sea.com on seasip.info -->

== Sample output ==
This is an example of CHECK2 checksums.

[[File:8000_plus_pcw_basic_check2_sample.png|800px|alt=A dot-matrix printed program listing, with a column of checksums to the first. The first line is "140 check%=0 : j%=0" and has checksum "06BD".]]

== Links ==
* ''FIXME link to JCE's typeins.tar.gz on seasip.info'', a collection of all the type-in listings from ''8000/PCW Plus'', put together by John Elliott; the <tt>Checker</tt> directory contains all the versions of the BASIC checker program, a history, C versions, and the author's own CHECK5.