zx-generation/src/core/flags.js

A high-fidelity ZX Spectrum emulator in JavaScript — fully generated by a large language model (LLM) to explore the boundaries of AI in systems programming.

/**
 * Z80 Flags Manager
 * Handles all flag operations including undocumented F3/F5 flags
 */
class Flags {
    constructor() {
        this.masks = {
            S: 0x80, // Sign
            Z: 0x40, // Zero
            F5: 0x20, // Undocumented - copy of bit 5
            H: 0x10, // Half carry
            F3: 0x08, // Undocumented - copy of bit 3
            PV: 0x04, // Parity/Overflow
            N: 0x02, // Add/Subtract
            C: 0x01, // Carry
        };
    }

    /**
     * Get flag value from F register
     */
    getFlag(fRegister, flagMask) {
        return (fRegister & flagMask) !== 0;
    }

    /**
     * Set or clear a flag in F register
     */
    setFlag(fRegister, flagMask, value) {
        if (value) {
            return fRegister | flagMask;
        } else {
            return fRegister & ~flagMask;
        }
    }

    /**
     * Update flags after arithmetic/logical operations
     */
    updateFlags(fRegister, result, operation = 'arithmetic') {
        const result8 = result & 0xff;
        let newF = fRegister;

        // Always update undocumented flags F3 and F5
        newF = this.setFlag(newF, this.masks.F5, (result8 & 0x20) !== 0);
        newF = this.setFlag(newF, this.masks.F3, (result8 & 0x08) !== 0);

        newF = this.setFlag(newF, this.masks.S, (result8 & 0x80) !== 0);
        newF = this.setFlag(newF, this.masks.Z, result8 === 0);

        if (operation === 'subtract') {
            newF = this.setFlag(newF, this.masks.N, true);
        } else if (operation === 'arithmetic') {
            newF = this.setFlag(newF, this.masks.N, false);
        } else if (operation === 'logical') {
            newF = this.setFlag(newF, this.masks.N, false);
            // For logical operations, set parity flag
            newF = this.setFlag(newF, this.masks.PV, this.calculateParity(result8));
        }

        return newF;
    }

    /**
     * Calculate parity of an 8-bit value
     */
    calculateParity(value) {
        let parity = 0;
        let temp = value & 0xff;
        for (let i = 0; i < 8; i++) {
            if (temp & 1) {
                parity++;
            }
            temp >>= 1;
        }
        return (parity & 1) === 0;
    }

    /**
     * Update flags for IN instruction
     */
    updateInFlags(fRegister, value) {
        let newF = fRegister;

        newF = this.setFlag(newF, this.masks.S, (value & 0x80) !== 0);
        newF = this.setFlag(newF, this.masks.Z, value === 0);
        newF = this.setFlag(newF, this.masks.H, false);
        newF = this.setFlag(newF, this.masks.N, false);
        newF = this.setFlag(newF, this.masks.PV, this.calculateParity(value));

        // Undocumented flags
        newF = this.setFlag(newF, this.masks.F5, (value & 0x20) !== 0);
        newF = this.setFlag(newF, this.masks.F3, (value & 0x08) !== 0);

        return newF;
    }

    /**
     * Update flags for increment operation
     */
    updateIncFlags(fRegister, originalValue, result) {
        let newF = fRegister;

        newF = this.setFlag(newF, this.masks.S, (result & 0x80) !== 0);
        newF = this.setFlag(newF, this.masks.Z, result === 0);
        newF = this.setFlag(newF, this.masks.H, (originalValue & 0x0f) === 0x0f);
        newF = this.setFlag(newF, this.masks.PV, originalValue === 0x7f);
        newF = this.setFlag(newF, this.masks.N, false);

        // Undocumented flags
        newF = this.setFlag(newF, this.masks.F5, (result & 0x20) !== 0);
        newF = this.setFlag(newF, this.masks.F3, (result & 0x08) !== 0);

        return newF;
    }

    /**
     * Update flags for decrement operation
     */
    updateDecFlags(fRegister, originalValue, result) {
        let newF = fRegister;

        newF = this.setFlag(newF, this.masks.S, (result & 0x80) !== 0);
        newF = this.setFlag(newF, this.masks.Z, result === 0);
        newF = this.setFlag(newF, this.masks.H, (originalValue & 0x0f) === 0);
        newF = this.setFlag(newF, this.masks.PV, originalValue === 0x80);
        newF = this.setFlag(newF, this.masks.N, true);

        // Undocumented flags - FIXED: was this.flags.F3, now this.masks.F3
        newF = this.setFlag(newF, this.masks.F5, (result & 0x20) !== 0);
        newF = this.setFlag(newF, this.masks.F3, (result & 0x08) !== 0);

        return newF;
    }

    /**
     * Update flags for BIT test operation
     */
    updateBitTestFlags(fRegister, bit, value) {
        const mask = 1 << bit;
        const result = value & mask;
        let newF = fRegister;

        newF = this.setFlag(newF, this.masks.Z, result === 0);
        newF = this.setFlag(newF, this.masks.H, true);
        newF = this.setFlag(newF, this.masks.N, false);
        newF = this.setFlag(newF, this.masks.S, bit === 7 && result !== 0);
        newF = this.setFlag(newF, this.masks.PV, result === 0); // PV acts as Z for BIT

        // Undocumented flags: F3 and F5 are set from the value being tested
        newF = this.setFlag(newF, this.masks.F5, (value & 0x20) !== 0);
        newF = this.setFlag(newF, this.masks.F3, (value & 0x08) !== 0);

        return newF;
    }
}

export { Flags };