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74157: Prevent unnecessary reads (nw)

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This commit is contained in:
AJR 2017-06-12 22:48:46 -04:00
parent 56bd27a963
commit 771dcf836a
1 changed files with 5 additions and 5 deletions
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10
src/devices/machine/74157.cpp
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@ -54,7 +54,7 @@ void ls157_device::device_start()
// resolve callbacks
m_a_in_cb.resolve();
m_b_in_cb.resolve();
m_out_cb.resolve_safe();
m_out_cb.resolve();
// register items for save state
save_item(NAME(m_a));
@ -178,7 +178,7 @@ void ls157_device::write_a_bit(int bit, bool state)
else
m_a &= ~(1 << bit);
if (!m_strobe && !m_select)
if (!m_strobe && !m_select && !m_out_cb.isnull())
m_out_cb(m_a);
}
}
@ -202,7 +202,7 @@ void ls157_device::write_b_bit(int bit, bool state)
else
m_b &= ~(1 << bit);
if (!m_strobe && m_select)
if (!m_strobe && m_select && !m_out_cb.isnull())
m_out_cb(m_b);
}
}
@ -237,7 +237,7 @@ WRITE_LINE_MEMBER(ls157_device::strobe_w)
m_strobe = bool(state);
// Clear output when strobe goes high
if (m_strobe)
if (m_strobe && !m_out_cb.isnull())
m_out_cb(0);
else
update_output();
@ -254,7 +254,7 @@ void ls157_device::update_output()
{
// S high, strobe low: Y1-Y4 = B1-B4
// S low, strobe low: Y1-Y4 = A1-A4
if (!m_strobe)
if (!m_strobe && !m_out_cb.isnull())
{
if (m_select)
m_out_cb(m_b_in_cb.isnull() ? m_b : (m_b_in_cb() & 0xf));