Corrections and some added explanation.

1 files changed, 63 insertions, 56 deletions

diff --git a/cobol.html.markdown b/cobol.html.markdown
index b0ab5b6d..4452bd95 100644
--- a/cobol.html.markdown
+++ b/cobol.html.markdown
@@ -7,12 +7,13 @@ filename: learn.COB
 COBOL is a business-oriented language revised multiple times since its original design in 1960. It is claimed to still be used in over 80% of 
 organizations.
 
-```
+```cobol
       *COBOL. Coding like it's 1985.
+      *Compiles with GnuCOBOL in OpenCobolIDE 4.7.6.
        
       *COBOL has significant differences between legacy (COBOL-85)
       *and modern (COBOL-2002 and COBOL-2014) versions.
-      *Legacy versions require columns 1-6 to be blank (they are used 
+      *Legacy versions require columns 1-6 to be blank (they are used
       *to store the index number of the punched card..)
       *A * in column 7 means a comment.
       *In legacy COBOL, a comment can only be a full line.
@@ -21,9 +22,9 @@ organizations.
       *Legacy COBOL also imposes a limit on maximum line length.
       *Keywords have to be in capitals in legacy COBOL,
       *but are case insensitive in modern.
-              
+
       *First, we must give our program ID.
-      *Identification division can include other values too, 
+      *Identification division can include other values too,
       *but they are comments only. Program-id is mandatory.
        identification division.
        program-id. learn.
@@ -32,7 +33,7 @@ organizations.
        data division.
        working-storage section.
 
-      *Variables are specified by a "picture" - how they should be 
+      *Variables are specified by a "picture" - how they should be
       *displayed, and variable type is inferred from this.
       *The "01" value is the level number which is used for building
       *data structures.
@@ -40,9 +41,9 @@ organizations.
        01  age picture 999.   *> A number up to 3 digits.
        01  valx picture 999.  *> Another number up to 3 digits.
        01  inyear picture s9(7). *> S makes number signed.
-                                 *> Brackets indicate 6 repeats of 9,
+                                 *> Brackets indicate 7 repeats of 9,
                                  *> ie a 6 digit number (not an array).
-       
+
       *Now let's write some code.
        procedure division.
 
@@ -64,98 +65,104 @@ organizations.
 
             display age *> Left-padded to three chracaters with zeroes,
                         *> because of the defined PICTURE for age.
-            
+
             *> We have two ways of doing a FOR loop.
             *> Old style way: doesn't give an index.
             perform age times
                 display "*" with no advancing *> Ie, no newline at end
-            end-perform.
+            end-perform
             display "." *> Output buffer isn't flushed until newline.
-            
+
             *> New style way: with an index.
-            perform varying valx from 1 by 1 until valx > age 
-                display valx "-" with no advancing 
-            end-perform.
+            perform varying valx from 1 by 1 until valx > age
+                display valx "-" with no advancing
+            end-perform
             display "."
-            
+
             *> If tests are still good old if tests.
-            if myname = "Bob" then  
+            if myname = "Bob" then
                 display "I don't like Bob."
-            else 
-                display "I don't know you." 
-            end-if 
-            
-            *> There are two ways of doing subprograms and calling 
+            else
+                display "I don't know you."
+            end-if
+
+            *> There are two ways of doing subprograms and calling
             *> them.
             *> The simplest way: a paragraph.
             perform subparagraph
-            
+
             *> The complex way, with parameters and stuff.
-            call "eratosthenes" using age returning valx 
-            
+            call "eratosthenes" using age returning valx
+
             display "There were " valx " primes."
-            
+
             stop run.
 
        subparagraph. *> Marks the top of an internal subprogram.
            *> Shares variable score with its caller.
-           accept inyear from day yyyyddd.
            
+           *> Read year from system timer.
+           *> Remember the whole "year 2000 crisis"? The yyyyddd 
+           *> option was added in response to that.
+           accept inyear from day yyyyddd.
+
            *> We can do math step-by-step like this...
-           divide 1000 into inyear. 
+           divide 1000 into inyear.
            subtract age from inyear.
-           
+
            display "You were born in " inyear "."
-           
+
            *> Or we can just use expressions.
            compute inyear = 1970 - inyear.
-           
-           *> Note: if inyear has gone negative, its negativity will
-           *> not appear when printed, because we didn't include an 
-           *> S (sign character) in the PICTURE.
-           if inyear >= 0 then 
+
+           if inyear >= 0 then
                display "When you were " inyear ", " with no advancing
            else
                display inyear " years before you were born, " with no
                  advancing
            end-if
-               
-           display "COBOL was the most popular language in the world."         
-           .
-           
-           
+
+           display "COBOL was the most popular language in the world."
+           . *> You can put the final . on a new line if it's clearer.
+
+
+      *If we want to use a subprogram, we use literally a subprogram.
+      *This is the entire program layout, repeated for the 
+      *eratosthenes subroutine.
        identification division.
        program-id. eratosthenes.
 
        data division.
        working-storage section.
-      *Declare an array. 
-      *We can declare a variable to use as an index for it at the 
+      *Declare an array.
+      *We can declare a variable to use as an index for it at the
       *same time.
        01  sieve pic 9 occurs 999 times indexed by sa, sb.
+           *> Standard cobol doesn't have a boolean type.
        01  pstart pic 999.
        01  counter pic 999.
-            
+
       *Our parameters have to be declared in the linkage section.
+      *Their pictures must match the values they're called with.
        linkage section.
        01  maxvalue picture 999.
-       
-      *"using" declares our actual parameter variables. 
+
+      *"using" declares our actual parameter variables.
       *"returning" declares the variable value returned at end.
        procedure division using maxvalue returning counter.
        main-procedure.
-           
+
            display "Here are all the primes up to " maxvalue "."
-           
+
            perform varying sa from 1 by 1 until sa > maxvalue
                move 1 to sieve (sa)
-           end-perform 
-               
+           end-perform
+
            perform varying sa from 2 by 1 until sa > maxvalue
-               if sieve(sa) = 1 then 
+               if sieve(sa) = 1 then
                    compute pstart = sa + sa
-                   perform varying sb from pstart by sa until sb > 
-                   maxvalue 
+                   perform varying sb from pstart by sa until sb >
+                   maxvalue
                        move 0 to sieve(sb)
                    end-perform
                end-if
@@ -163,17 +170,17 @@ organizations.
 
            initialise counter  *> To zero by default for a number.
 
-           perform varying sa from 2 by 1 until sa > maxvalue 
+           perform varying sa from 2 by 1 until sa > maxvalue
                if sieve(sa) = 1 THEN
                    display sa
                    add 1 to counter
-               end-if 
+               end-if
            end-perform.
-           
-       end program eratosthenes.     
-           
+
+       end program eratosthenes.
+
        end program learn.
-       
+
 ```
 
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