The Language Reference

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The reference manual for nel, a language which is intended to run on the original Nintendo Entertainment System. This language discards the mnemonics and restricted syntax of other 6502 assemblers and replaces it with a new syntax, while still mapping to the same 6502 machine code. It gives the direct control like any other assembly language, but with the syntactic flavoring of a higher-level language.

nel changes syntax for the better. It introduces language keywords and new syntactic constructs, instead of relying on dot-prefixed .directives. It no longer mandates obscure whitespace rules. It combines similar 6502 instructions into condensed groups (this: get src, instead of lda, ldx, ldy, tax, txs, and so forth). The result is more legible assembly.

Table of Contents

• Directives
  • The Header Directive, using the ines keyword to setup the output file.
  • Relocation Directives, using the in keyword to layout different parts of the program.
  • Inclusion Directives, using the include keyword to require an external source file.
• Definitions
  • Bank Definitions, using the bank keyword to declare sections of RAM, PRG ROM and CHR ROM.
  • Label Definitions, using the def keyword to point to a particular section.
  • Constant Definitions, using the let keyword.
  • Enumeration Definitions, using the enum keyword to declare several sequential constants.
  • Variable Definitions, using the var keyword.
  • Scoping, using a begin / end block.
  • Package Definitions, using a package block.
• Statements
  • Branches, using goto and goto ... when statements.
  • Subroutines, using call and return statements.
  • Interrupts, setting up NES's interrupt vectors and using resume statements.
  • Instructions, of form this: command, command, ...
  • Data Statements, for emitting byte or word data into the code.
  • Binary Embeds, by using the embed keyword to insert a binary file verbatim.
  • Empty Statements, using a semi-colon ;.
  • If Statements, using if ... then / elseif / else for conditional compilation on constant expressions, and a convenient shorthand for runtime branching on flags.
  • While Statements, using while as a shorthand for loops done zero or more times.
  • Repeat Statements, using repeat as a shorthand for loops done one or more times, and for infinite loops.
• Miscellaneous
  • Integer Literals for decimal, hexadecimal or binary integers.
  • String Literals for textual data.
  • Comments for documenting and debugging your code.
  • Constant Expressions for combining terms and literals.
  • Registers & Memory Addressing Modes which affect how an expression should be interpreted in regards to an instruction.
  • Processor Status Flags for use in various conditional branches and instructions.