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SPECIFICS.md

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ABOUT

The following discussion is with respect to the spike5*.txt specification under the folder spikes.

SPECIFICATION ISSUES

The following issues were noted during project development, presented along with the chosen resolutions:

  • ISSUE #1: The spec doesn't say much about comma operator type rules and usage.
  • RESOLUTION: Comma operator, doesn't preserve the lvalue and is neither assignable according to the implementation. Check 'tests/e10.loboc' for more info.

IMPLEMENTATION SPECIFICS

The enclosed loboc program is a fully-conforming implementation of the spike5*.txt specification, passing all supplied tests.

  • SPECIFIC #1: In case any errors apart from runtime errors are encountered, the assembly code is not printed, instead spike4-output with error messages are shown to help understand the errors.

  • SPECIFIC #2: Pointer increment doesn't always increment the pointer by 1 but by the size of the type it's pointing at. Same thing happens in the case of pointer decrement. Check 'tests/is5_pointer.loboc' for more info.

ISEQ5 IMPLEMENTATION SPECIFICS

ISEQ5 is an extended feature using which the program can now accept input and produce output. This is made possible with the help of a special keyword "loboc". Below are the main use cases along with the description. Check grammar.txt under spike5 folder for the complete usage.

Command Description
loboc >> a; Takes the input and stores it in the variable 'a'.
loboc << a; Prints the value of a.

Some key points to note here:-

  • The type of the variable 'a' has to be either signed, unsigned or bool.

  • Variable 'a' could be an element of an array as well.

  • SPECIFIC #1: ISEQ5 is backwards-compatible. Hence, no separate mechanism is required to build it. To test ISEQ5, the programmer first has to run ./loboc compiler to generate the assembly code and then run this assembly code through spim without using the TLC. Consider the following example for a better understanding.

    $> make
$> ./loboc tests/is5_bool.loboc > is5_bool.s
$> spim -file is5_bool.s

  • SPECIFIC #2: Although the defined grammar for IO_EXPR allows a wide variety of usage of the keyword "loboc", not everything is valid and the program might return an error to help programmer understand the limitations. Semantic check narrows down the usage as well. Check is5*.loboc under tests folder for some valid test cases.

  • SPECIFIC #3: When input for a bool type variable is taken from the user, the input is converted to either 0 or 1. In the case of a non-zero input, the input is converted to 1, otherwise it stays as it is i.e. 0. Check is5_bool.loboc under tests folder and run it for a demonstration.

  • SPECIFIC #4: Unsigned integer can take signed input but it will still be treated as unsigned integer throughout the program except when it's printed because MIPS doesn't support a system call to print an unsigned integer. Therefore, all integers are printed as signed integers. Check 'is5_signed_unsigned.loboc' under tests folder for more info.

  • SPECIFIC #5: For better console readability, a new line is printed every time the program produces any output.