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GETREG_SETREG_IMPLEMENTATION.md

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GETREG_SETREG_IMPLEMENTATION.md

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GetReg and SetReg Built-in Functions Implementation

Overview

Added two new built-in functions to the HAS compiler for direct register access:

  1. GetReg(register) - Reads a value from a specified register
  2. SetReg(register, value) - Writes a value to a specified register

Syntax

GetReg

var result: long = GetReg("d0");  // Read from data register d0
var addr: long = GetReg("a1");    // Read from address register a1

SetReg

SetReg("d3", myvar);              // Write variable to data register d3
SetReg("a0", address_value);      // Write address to address register a0
SetReg("d2", x + y);              // Write expression result to register

Valid Registers

Data Registers

  • d0, d1, d2, d3, d4, d5, d6, d7 (8 data registers)

Address Registers

  • a0, a1, a2, a3 (4 address registers)

Note: Registers a4-a7 are not valid parameters as they are reserved:

  • a4: Frame pointer (in procedures with local variables)
  • a5: Caller-save area
  • a6: Additional uses in some calling conventions
  • a7: Stack pointer

Type System

  • GetReg always returns long (4-byte value)
  • SetReg accepts any expression that evaluates to a value

Implementation Details

Parser Changes (parser.py)

  • Added getreg and setreg grammar rules in the atom section
  • Parses string literals for register names: GetReg("d0") → ast.GetReg(register="d0")
  • Parses SetReg with two arguments: SetReg("d3", expr) → ast.SetReg(register="d3", value=expr)

AST Nodes (ast.py)

@dataclass
class GetReg:
    register: str  # Register name (d0-d7, a0-a3)

@dataclass
class SetReg:
    register: str  # Register name (d0-d7, a0-a3)
    value: Any     # Expression to assign to register

Validation (validator.py)

  • Validates that register names are in the allowed set {d0-d7, a0-a3}
  • GetReg: No expression validation needed (just register validation)
  • SetReg: Validates both register and the value expression

Code Generation (codegen.py)

  • GetReg: Generates move.l <register>,<target_reg> to read register into target
  • SetReg:
    1. Evaluates the value expression into a temp register (d1 or d2)
    2. Generates move.l <temp_reg>,<register> to write to target register

Usage Examples

Example 1: Simple register read/write

proc example1() -> long {
    var myval: long = 42;
    SetReg("d4", myval);          // Write to d4
    var result: long = GetReg("d4");  // Read from d4
    return result;
}

Example 2: Working with address registers

proc example2() -> long {
    var ptr: long = 0x1000;
    SetReg("a0", ptr);              // Load pointer into a0
    var ptr_value: long = GetReg("a0");  // Read it back
    return ptr_value;
}

Example 3: SetReg with expressions

proc example3() -> long {
    var a: long = 10;
    var b: long = 20;
    SetReg("d5", a + b);            // Write expression result to d5
    var result: long = GetReg("d5");    // Read it back
    return result;
}

Generated Assembly Examples

GetReg Example

; var result: long = GetReg("d0");
move.l d0,-4(a4)      ; Read from d0, store in local variable

SetReg Example

; SetReg("d3", myval);
move.l -4(a4),d1      ; Load myval into d1
move.l d1,d3          ; Write d1 to d3

SetReg with Expression

; SetReg("d2", a + b);
move.l -4(a4),d1      ; Load a
move.l -8(a4),d2      ; Load b
add.l d2,d1           ; Add: d1 = a + b
move.l d1,d2          ; Write result to d2

Test Files

Created example files to test the implementation:

  • examples/getreg_setreg_simple.has - Basic GetReg/SetReg usage
  • examples/getreg_setreg_test.has - Comprehensive tests with all registers
  • examples/getreg_addr_regs.has - Tests address registers a0-a3
  • examples/getreg_invalid_reg.has - Validation error test (attempts to use a4)

Error Handling

Invalid register names are caught during validation:

Validation error: GetReg invalid register 'a4'. Valid registers: d0-d7, a0-a3

Notes

  • GetReg/SetReg are expressions, not statements (they can be used in assignments)
  • SetReg returns a value for compatibility with expression context (the written value)
  • Register values are not type-checked - they're treated as raw long values
  • These functions provide low-level register access for performance-critical code and hardware interaction