Memory

The Xbox has 64MB Memory. This could be expanded to 128MB of memory on boards of revision 1.0-1.4 (Boards of revision 1.0-1.4 have empty spots for the extra memory, but they were later removed on 1.6 boards) but no games took advantage of it. The debug Xbox and the Chihiro both contained 128MB Memory.

The memory was shared between the CPU and GPU. On the retail Xbox, the Flash ROM and MCPX ROM are also mapped to memory at the top 16 MiB and the top 512 Bytes respectively. However on Debug Xboxes and Chihiro, only the Flash is mapped as they don't contain an MCPX ROM.

Memory Type	Retail Xbox Range	Debug/Chihiro Range
Main Memory	0x00000000 - 0x03FFFFFF	0x00000000 - 0x07FFFFFF
GPU (NV2A) Registers	0xFD000000 - 0xFDFFFFFF
APU Registers	0xFE800000 - 0xFE87FFFF
ACI (AC97) Registers	0xFEC00000 - 0xFEC00FFF
USB 0 Registers	0xFED00000 - 0xFED00FFF
USB 1 Registers	0xFED08000 - 0xFED08FFF
NIC (NVNet) Registers	0xFEF00000 - 0xFEF003FF
Flash ROM	0xFF000000 - 0xFFFFFFFF
MCPX ROM	0xFFFFFE00 - 0xFFFFFFFF	N/A

Code for emulating the memory might consist of:

#ifdef DEBUG || CHIHIRO
#define MEMORY_SIZE 128 * 1024 * 1024
int mcpx_active = 0;
#else
#define MEMORY_SIZE 64 * 1024 * 1024
int mcpx_active = 1;
#endif

#define FLASH_SIZE 256 * 1024
#define FLASH_MAP_SIZE 16 * 1024 * 1024
#define FLASH_MAP_ADDRESS (0xFFFFFFFF - FLASH_MAP_SIZE + 1)

#define MCPX_SIZE   0x200
#define MCPX_MAP_ADDRESS (0xFFFFFFFF - MCPX_MAP_SIZE + 1)

uint8_t memory[MEMORY_SIZE] = {0};
uint8_t flash[FLASH_SIZE] = {0};
uint8_t mcpx[MCPX_SIZE] = {0};

uint8_t get_memory_byte(uint32_t location) {
    if (location < MEMORY_SIZE) {
        return memory[location];
    }

    if (mcpx_active && location >= MCPX_MAP_ADDRESS) {
        return mcpx[location - MCPX_MAP_ADDRESS];
    }

    if (location >= FLASH_MAP_ADDRESS) {
        return flash[(location - FLASH_MAP_ADDRESS) % FLASH_SIZE];
    }

    printf("Memory in unspecified range: %08X\n", location);
    return 0;
}

uint16_t get_memory_word(uint32_t location) {
    return get_memory_byte(location + 1) << 8 | get_memory_byte(location);
}

uint32_t get_memory_dword(uint32_t location) {
    return get_memory_word(location + 2) << 16 | get_memory_word(location);
}

void deactivate_mcpx() {
    mcpx_active = 0;
}