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Let’s do a code ‘walk-through’

Let’s do a code ‘walk-through’. Here we examine the design details for a short, but not trivial, assembly language example. A good way to get us going…. It’s a short program (could fit on one page) It’s ‘self-contained’ (i.e., needs no libraries) It displays output that is helpful for CS

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Let’s do a code ‘walk-through’

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  1. Let’s do a code ‘walk-through’ Here we examine the design details for a short, but not trivial, assembly language example

  2. A good way to get us going… • It’s a short program (could fit on one page) • It’s ‘self-contained’ (i.e., needs no libraries) • It displays output that is helpful for CS • It illustrates important assembly principles • It implements a ‘nested loop’ algorithm

  3. Source-statement format • Each statement in an assembly language program’s source-file follows a ‘standard’ format, comprised of four distinct fields: label: opcode operand(s) # comment comma-separated list whitespace hashmark colon

  4. Labels • The ‘label’ field is allowed to be blank if the statement doesn’t need a label (i.e., if it’s not referred to by any other statements) • Where a label is needed, the programmer is responsible to making up the name: • It cannot begin with a digit-character (‘0’..’9’) • It can only contain letters and digits, or the three special characters: ‘.’, ‘_’, ‘$’

  5. Two kinds of ‘opcodes’ • The ‘opcode field’ holds a reserved word • An Intel-x86 ‘instruction’, or • A GNU-assembler ‘directive’ • This distinction will be very important! • But it’s easy to recognize the ‘directives’ because they begin with a dot-symbol (‘.’)

  6. Operands vary with the opcode • Some opcodes require no operands • Some opcodes need only one operand • Most opcodes need two operands • A few opcodes need three operands • When more than one operand is needed, the comma-symbol is used a separator

  7. Comment-field • The comments are programmer-designed • Their intent is to help other programmers understand the purpose of a statement in the overall flow of the program-algorithm • They are ignored by the assembler as far as the language-translation is concerned • But they will be displayed in an assembler listing-file (if in fact one is generated)

  8. Typical ‘counted-loop’ construct Initialize some variables Perform a loop-iteration Adjust the iteration-counter Done? no yes

  9. A loop-within-a-loop ‘outer’ loop ‘inner’ loop

  10. Demo: ‘nybbles.s’ • You can download this source-file from our class website to your local directory: $ cp /home/web/cruse/cs210/nybbles . • You can assemble it and link it, like this: $ as nybbles.s -o nybbles.o $ ld nybbles.o -o nybbles • Then you can execute it, like this: $ ./nybbles

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