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mc146818: support direct-access bus hookup method. [R. Belmont]

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This chip has 8 address/data lines and an address/data strobe.  Most
users hooked it up so the low-order address bit is the address/data
strobe, allowing you to write an address and then read/write data
at an adjacent address.

But if you don't mind using a few more gates you can latch the
low-order address bits from the CPU into the chip then latch the
data in or out of it so it works just like a RAM.  The MIPS DECstations
did this in their I/O ASICs.
This commit is contained in:
arbee 2018-07-15 13:07:51 -04:00
parent 7eff013f87
commit 16336848af
2 changed files with 82 additions and 65 deletions
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33
src/devices/machine/mc146818.cpp
View File

@ -520,7 +520,18 @@ READ8_MEMBER( mc146818_device::read )
break;
case 1:
switch (m_index)
data = read_direct(space, m_index);
break;
}
return data;
}
READ8_MEMBER( mc146818_device::read_direct )
{
uint8_t data = 0;
switch (offset)
{
case REG_A:
data = m_data[REG_A];
@ -548,23 +559,18 @@ READ8_MEMBER( mc146818_device::read )
data = m_data[m_index];
break;
}
break;
}
LOG("mc146818_port_r(): index=0x%02x data=0x%02x\n", m_index, data);
LOG("mc146818_port_r(): offset=0x%02x data=0x%02x\n", offset, data);
return data;
}
//-------------------------------------------------
// write - I/O handler for writing
//-------------------------------------------------
WRITE8_MEMBER( mc146818_device::write )
{
LOG("mc146818_port_w(): index=0x%02x data=0x%02x\n", m_index, data);
switch (offset)
{
case 0:
@ -572,7 +578,16 @@ WRITE8_MEMBER( mc146818_device::write )
break;
case 1:
switch (m_index)
write_direct(space, m_index, data);
break;
}
}
WRITE8_MEMBER( mc146818_device::write_direct )
{
LOG("mc146818_port_w(): offset=0x%02x data=0x%02x\n", offset, data);
switch (offset)
{
case REG_SECONDS:
// top bit of SECONDS is read only
@ -605,6 +620,4 @@ WRITE8_MEMBER( mc146818_device::write )
m_data[m_index] = data;
break;
}
break;
}
}

4
src/devices/machine/mc146818.h
View File

@ -72,6 +72,10 @@ public:
DECLARE_READ8_MEMBER( read );
DECLARE_WRITE8_MEMBER( write );
// direct-mapped read/write access
DECLARE_READ8_MEMBER( read_direct );
DECLARE_WRITE8_MEMBER( write_direct );
protected:
mc146818_device(const machine_config &mconfig, device_type type, const char *tag, device_t *owner, uint32_t clock);