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spg2xx: Correct PRNG emulation. [Ryan Holtz]

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This commit is contained in:
MooglyGuy 2019-01-27 21:34:29 +01:00 committed by Vas Crabb
parent 76323eb770
commit 62ba8c558c
1 changed files with 50 additions and 4 deletions
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54
src/devices/machine/spg2xx.cpp
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@ -237,6 +237,9 @@ void spg2xx_device::device_reset()
m_timer_b_tick_rate = 0;
m_io_regs[0x23] = 0x0028;
m_io_regs[0x2c] = 0x1418;
m_io_regs[0x2d] = 0x1658;
m_uart_rx_available = false;
memset(m_uart_rx_fifo, 0, ARRAY_LENGTH(m_uart_rx_fifo));
m_uart_rx_fifo_start = 0;
@ -1022,6 +1025,38 @@ void spg2xx_device::uart_rx(uint8_t data)
}
}
static uint16_t prng_mul(uint16_t a, uint16_t b)
{
uint16_t d = 0;
if (a >= 0x8000) a &= 0x7fff;
if (b >= 0x8000) b &= 0x7fff;
for (int i = 0; i < 16; i++)
{
d = (d > 0x4000) ? ((d << 1) - 0x8000) : (d << 1);
if (a & 0x8000)
d += b;
if (d >= 0x8000)
d -= 0x8000;
a <<= 1;
}
return d;
}
static uint16_t prng_pow(uint16_t value, uint16_t exponent)
{
uint16_t r = 1;
while (exponent > 0)
{
if (exponent & 1)
{
r = prng_mul(r, value);
}
exponent >>= 1;
value = prng_mul(value, value);
}
return r;
}
READ16_MEMBER(spg2xx_device::io_r)
{
static const char *const gpioregs[] = { "GPIO Data Port", "GPIO Buffer Port", "GPIO Direction Port", "GPIO Attribute Port", "GPIO IRQ/Latch Port" };
@ -1096,10 +1131,21 @@ READ16_MEMBER(spg2xx_device::io_r)
LOGMASKED(LOG_IO_READS, "io_r: NTSC/PAL = %04x\n", m_pal_flag);
return m_pal_flag;
case 0x2c: case 0x2d: // PRNG 0/1
val = machine().rand() & 0x0000ffff;
LOGMASKED(LOG_IO_READS, "io_r: PRNG %d = %04x\n", offset - 0x2c, val);
break;
case 0x2c: // PRNG 0
{
const uint16_t value = m_io_regs[0x2c];
const uint16_t pow14 = prng_pow(value, 14);
m_io_regs[0x2c] = (pow14*value + pow14 + 1) & 0x7fff;
return value;
}
case 0x2d: // PRNG 1
{
const uint16_t value = m_io_regs[0x2d];
const uint16_t pow14 = prng_pow(value, 14);
m_io_regs[0x2d] = (pow14*value + pow14 + 1) & 0x7fff;
return value;
}
case 0x2e: // FIQ Source Select
LOGMASKED(LOG_FIQ, "io_r: FIQ Source Select = %04x\n", val);