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DMA ports 81h and 82h workaround for proper CPU speed detection on the MegaPC BIOS and limited the MegaPC CPU speeds to >= 16 MHz and <= 25 MHz.

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OBattler
2024-02-02 17:52:00 +01:00
parent 285accae5b
commit 9e7fc6a60d
2 changed files with 40 additions and 10 deletions
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46
src/dma.c
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@@ -949,16 +949,46 @@ dma_page_read(uint16_t addr, UNUSED(void *priv))
uint8_t convert[8] = CHANNELS;
uint8_t ret = 0xff;
addr &= 0x0f;
ret = dmaregs[2][addr];
if (((addr & 0xfffc) == 0x80) && (CS == 0xf000) &&
((cpu_state.pc & 0xfffffff8) == 0x00007278) &&
!strcmp(machine_get_internal_name(), "megapc")) switch (addr) {
/* The Amstrad MegaPC Quadtel BIOS times a sequence of:
mov ax,di
div bx
And expects this value to be at least 0x06e0 for 20 MHz,
and at least 0x0898 for 25 MHz, everything below 0x06e0
is assumed to be 16 MHz. Given that for some reason, this
does not occur on 86Box, we have to work around it here,
we return 0x0528 for 16 MHz, because it's the next in the
mathematical sequence (it equals 0x06e0 - (0x0898 - 0x06e0)). */
case 0x0081:
if (cpu_busspeed >= 25000000)
ret = 0x98;
else if (cpu_busspeed >= 20000000)
ret = 0xe0;
else
ret = 0x28;
break;
case 0x0082:
if (cpu_busspeed >= 25000000)
ret = 0x08;
else if (cpu_busspeed >= 20000000)
ret = 0x06;
else
ret = 0x05;
break;
} else {
addr &= 0x0f;
ret = dmaregs[2][addr];
if (addr >= 8)
addr = convert[addr & 0x07] | 4;
else
addr = convert[addr & 0x07];
if (addr >= 8)
addr = convert[addr & 0x07] | 4;
else
addr = convert[addr & 0x07];
if (addr < 8)
ret = dma[addr].page_l;
if (addr < 8)
ret = dma[addr].page_l;
}
return ret;
}

4
src/machine/machine_table.c
View File

@@ -4628,8 +4628,8 @@ const machine_t machines[] = {
.cpu = {
.package = CPU_PKG_386SX,
.block = CPU_BLOCK_NONE,
.min_bus = 0,
.max_bus = 0,
.min_bus = 16000000,
.max_bus = 25000000,
.min_voltage = 0,
.max_voltage = 0,
.min_multi = 0,