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-spg2xx: More correct PRNG emulation, nw

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MooglyGuy 2019-01-27 21:54:17 +01:00 committed by Vas Crabb
parent 62ba8c558c
commit dda8bad8aa
1 changed files with 12 additions and 36 deletions
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48
src/devices/machine/spg2xx.cpp
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@ -1025,38 +1025,6 @@ 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) 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" }; static const char *const gpioregs[] = { "GPIO Data Port", "GPIO Buffer Port", "GPIO Direction Port", "GPIO Attribute Port", "GPIO IRQ/Latch Port" };
@ -1134,16 +1102,14 @@ READ16_MEMBER(spg2xx_device::io_r)
case 0x2c: // PRNG 0 case 0x2c: // PRNG 0
{ {
const uint16_t value = m_io_regs[0x2c]; const uint16_t value = m_io_regs[0x2c];
const uint16_t pow14 = prng_pow(value, 14); m_io_regs[0x2c] = ((value << 1) | (BIT(value, 14) ^ BIT(value, 13))) & 0x7fff;
m_io_regs[0x2c] = (pow14*value + pow14 + 1) & 0x7fff;
return value; return value;
} }
case 0x2d: // PRNG 1 case 0x2d: // PRNG 1
{ {
const uint16_t value = m_io_regs[0x2d]; const uint16_t value = m_io_regs[0x2d];
const uint16_t pow14 = prng_pow(value, 14); m_io_regs[0x2d] = ((value << 1) | (BIT(value, 14) ^ BIT(value, 13))) & 0x7fff;
m_io_regs[0x2d] = (pow14*value + pow14 + 1) & 0x7fff;
return value; return value;
} }
@ -1712,6 +1678,16 @@ WRITE16_MEMBER(spg2xx_device::io_w)
break; break;
} }
case 0x2c: // PRNG 0 seed
LOGMASKED(LOG_IO_WRITES, "io_w: PRNG 0 seed = %04x\n", data & 0x7fff);
m_io_regs[offset] = data & 0x7fff;
break;
case 0x2d: // PRNG 1 seed
LOGMASKED(LOG_IO_WRITES, "io_w: PRNG 1 seed = %04x\n", data & 0x7fff);
m_io_regs[offset] = data & 0x7fff;
break;
case 0x2e: // FIQ Source Select case 0x2e: // FIQ Source Select
{ {
static const char* const s_fiq_select[8] = static const char* const s_fiq_select[8] =