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 map

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

I/O port map

^[FIXME]

Ports	Purpose
0000-001f	dma1
0020-003f	pic1
0040-0043	timer0
0050-0053	timer1
0060-006f	keyboard
0070-007f	rtc
0080-008f	dma page reg
00a0-00bf	pic2
00c0-00df	dma2
00f0-00ff	fpu
01f0-01f7	ide0
03c0-03df	vesafb
03f6-03f6	ide0
0cf8-0cff	PCI conf1
1000-100f	nVidia Corporation nForce PCI System Management
1080-10ff	nVidia Corporation nForce MC'97 Modem (Intel 537)
1400-14ff	nVidia Corporation nForce MC'97 Modem (Intel 537)
c000-c00f	nVidia Corporation nForce PCI System Management (amd756-smbus)
c200-c21f	nVidia Corporation nForce PCI System Management
d000-d0ff	nVidia Corporation nForce AC'97 Audio
d200-d27f	nVidia Corporation nForce AC'97 Audio
e000-e007	nVidia Corporation nForce Ethernet Controller
ff60-ff6f	nVidia Corporation nForce IDE

Emulation

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;
}