Editing Burroughs Large Systems (section)

=== How programs map to the stack ===

Here is an example of how programs map to the stack structure

 '''begin'''
    — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —
    — This is lexical level 2 (level zero is reserved for the operating system and level 1 for code segments).
    — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —
    
    — At level 2 we place global variables for our program.
    
    '''integer''' ''i'', ''j'', ''k'';
    '''real''' ''f'', ''g'';
    '''array''' ''a'' [0:9];
    
    '''procedure''' ''p'' ('''real''' ''p1'', ''p2'');
       '''value''' ''p1'';     — p1 passed by value, p2 implicitly passed by reference.
       '''begin'''
          — — — — — — — — — — — — — — — — — —
          — This block is at lexical level 3
          — — — — — — — — — — — — — — — — — —
          '''real''' ''r1'', ''r2'';<br />
          ''r2'' := ''p1'' * ''5'';
          ''p2'' := ''r2'';  — This sets ''g'' to the value of ''r2''
          ''p1'' := ''r2'';  — This sets ''p1'' to ''r2'', but not ''f''
          — Since this overwrites the original value of ''f'' in ''p1'' it ''might'' be a
          — coding mistake.  Some few of ALGOL's successors therefore insist that
          — value parameters be read only – but most do not.
          
          '''if''' ''r2'' > ''10'' '''then'''
             '''begin'''
                — — — — — — — — — — — — — — — — — — — — — — — — — — — —
                — A variable declared here makes this lexical level 4
                — — — — — — — — — — — — — — — — — — — — — — — — — — — —
                '''integer''' ''n'';<br />
                — The declaration of a variable makes this a block, which will invoke some
                — stack building code. Normally you won't declare variables here, in which
                — case this would be a compound statement, not a block.
                
                ... <== sample stack is executing somewhere here.
             '''end''';
       '''end''';
       
    .....
    
    ''p'' (''f'', ''g'');
 '''end'''.

Each stack frame corresponds to a lexical level in the current execution environment. As you can see, lexical level is the static textual nesting of a program, not the dynamic call nesting. The visibility rules of ALGOL, a language designed for single pass compilers, mean that only variables declared before the current position are visible at that part of the code, thus the requirement for forward declarations. All variables declared in enclosing blocks are visible. Another case is that variables of the same name may be declared in inner blocks and these effectively hide the outer variables which become inaccessible.

Lexical nesting is static, unrelated to execution nesting with recursion, etc. so it is very rare to find a procedure nested more than five levels deep, and it could be argued that such programs would be poorly structured. B5000 machines allow nesting of up to 32 levels. This could cause difficulty for some systems that generated Algol source as output (tailored to solve some special problem) if the generation method frequently nested procedure within procedure.