Nes_Cpu.h

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// NES 6502 CPU emulator
 
// Game_Music_Emu https://bitbucket.org/mpyne/game-music-emu/
#ifndef NES_CPU_H
#define NES_CPU_H
 
#include "blargg_common.h"
 
typedef blargg_long nes_time_t; // clock cycle count
typedef unsigned nes_addr_t; // 16-bit address
enum { future_nes_time = INT_MAX / 2 + 1 };
 
class Nes_Cpu {
public:
        // Clear registers, map low memory and its three mirrors to address 0,
        // and mirror unmapped_page in remaining memory
        void reset( void const* unmapped_page = 0 );
        
        // Map code memory (memory accessed via the program counter). Start and size
        // must be multiple of page_size. If mirror is true, repeats code page
        // throughout address range.
        enum { page_size = 0x800 };
        void map_code( nes_addr_t start, unsigned size, void const* code, bool mirror = false );
        
        // Access emulated memory as CPU does
        uint8_t const* get_code( nes_addr_t );
        
        // 2KB of RAM at address 0
        uint8_t low_mem [0x800];
        
        // NES 6502 registers. Not kept updated during a call to run().
        struct registers_t {
                uint16_t pc;
                uint8_t a;
                uint8_t x;
                uint8_t y;
                uint8_t status;
                uint8_t sp;
        };
        registers_t r;
        
        // Set end_time and run CPU from current time. Returns true if execution
        // stopped due to encountering bad_opcode.
        bool run( nes_time_t end_time );
        
        // Time of beginning of next instruction to be executed
        nes_time_t time() const             { return state->time + state->base; }
        void set_time( nes_time_t t )       { state->time = t - state->base; }
        void adjust_time( int delta )       { state->time += delta; }
        
        nes_time_t irq_time() const         { return irq_time_; }
        void set_irq_time( nes_time_t );
        
        nes_time_t end_time() const         { return end_time_; }
        void set_end_time( nes_time_t );
        
        // Number of undefined instructions encountered and skipped
        void clear_error_count()            { error_count_ = 0; }
        unsigned long error_count() const   { return error_count_; }
        
        // CPU invokes bad opcode handler if it encounters this
        enum { bad_opcode = 0xF2 };
        
public:
        Nes_Cpu() { state = &state_; }
        enum { page_bits = 11 };
        enum { page_count = 0x10000 >> page_bits };
        enum { irq_inhibit = 0x04 };
private:
        struct state_t {
                uint8_t const* code_map [page_count + 1];
                nes_time_t base;
                int time;
        };
        state_t* state; // points to state_ or a local copy within run()
        state_t state_;
        nes_time_t irq_time_;
        nes_time_t end_time_;
        unsigned long error_count_;
        
        void set_code_page( int, void const* );
        inline int update_end_time( nes_time_t end, nes_time_t irq );
};
 
inline uint8_t const* Nes_Cpu::get_code( nes_addr_t addr )
{
        return state->code_map [addr >> page_bits] + addr
        #if !BLARGG_NONPORTABLE
                % (unsigned) page_size
        #endif
        ;
}
 
inline int Nes_Cpu::update_end_time( nes_time_t t, nes_time_t irq )
{
        if ( irq < t && !(r.status & irq_inhibit) ) t = irq;
        int delta = state->base - t;
        state->base = t;
        return delta;
}
 
inline void Nes_Cpu::set_irq_time( nes_time_t t )
{
        state->time += update_end_time( end_time_, (irq_time_ = t) );
}
 
inline void Nes_Cpu::set_end_time( nes_time_t t )
{
        state->time += update_end_time( (end_time_ = t), irq_time_ );
}
 
#endif