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Merge pull request #1614 from TomHarte/6845Reading

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Return 0 for write-only and nonexistent registers.
This commit is contained in:
Thomas Harte
2025-10-20 13:56:27 -04:00
committed by GitHub
parent bd5a2f240d 2eb94f1b66
commit 25e783ff2f
1 changed files with 9 additions and 2 deletions
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11
Components/6845/CRTC6845.hpp
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@@ -94,7 +94,13 @@ public:
if(selected_register_ == 16 || selected_register_ == 17) status_ &= ~0x40;
if(personality == Personality::UM6845R && selected_register_ == 31) return dummy_register_;
if(selected_register_ < 12 || selected_register_ > 17) return 0xff;
// Registers below 12 are write-only; no registers are defined above position 17
// (other than the UM6845R-specific test register as per above).
//
// Per the BBC Wiki, attempting to read such a register results in 0.
if(selected_register_ < 12 || selected_register_ > 17) return 0x00;
return registers_[selected_register_];
}
@@ -114,6 +120,7 @@ public:
case 6: layout_.vertical.displayed = value; break;
case 7: layout_.vertical.start_sync = value; break;
case 8:
printf("Interlace mode: %d", value & 3);
switch(value & 3) {
default: layout_.interlace_mode_ = InterlaceMode::Off; break;
case 0b01: layout_.interlace_mode_ = InterlaceMode::Sync; break;
@@ -156,7 +163,7 @@ public:
0x7f, // Start horizontal retrace.
0x1f, 0x7f, 0x7f,
0xff, 0x1f, 0x7f, 0x1f,
0xfc, 0x1f, 0x7f, 0x1f,
uint8_t(RefreshAddress::Mask >> 8), uint8_t(RefreshAddress::Mask),
uint8_t(RefreshAddress::Mask >> 8), uint8_t(RefreshAddress::Mask),
};