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Writing 0 to the 6551 ACIA's control register is not a program reset.

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So far as I can tell, ever since the first version of the ACIA emulation in
Symon, writing 0x00 to the control register has been interpreted as a request
to reset, rather than to actually set the control register to 0x00. This is
strange for a number of reasons:

- All-zeros is actually a very sensible value for the control register, and
  is in fact the hardware-reset default.
- I can't find any description of such behaviour in the 6551, W65C51S, or
  W65C51N data sheets.
- The 6551 already has a way to trigger "program reset" by writing to the
  status register.

So I've removed that quirk, and writing to the control register now just
writes to the control register and nothing else.
This commit is contained in:
Tim Allen 2023-05-29 22:51:58 +10:00 committed by Seth Morabito
parent 5df775bbb0
commit 4423623816
1 changed files with 53 additions and 60 deletions
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113
src/main/java/com/loomcom/symon/devices/Acia6551.java
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@ -118,67 +118,60 @@ public class Acia6551 extends Acia {
controlRegister = data;
int rate = 0;
// If the value of the data is 0, this is a request to reset,
// otherwise it's a control update.
if (data == 0) {
reset();
} else {
// Mask the lower four bits to get the baud rate.
int baudSelector = data & 0x0f;
switch (baudSelector) {
case 0:
rate = 0;
break;
case 1:
rate = 50;
break;
case 2:
rate = 75;
break;
case 3:
rate = 110; // Real rate is actually 109.92
break;
case 4:
rate = 135; // Real rate is actually 134.58
break;
case 5:
rate = 150;
break;
case 6:
rate = 300;
break;
case 7:
rate = 600;
break;
case 8:
rate = 1200;
break;
case 9:
rate = 1800;
break;
case 10:
rate = 2400;
break;
case 11:
rate = 3600;
break;
case 12:
rate = 4800;
break;
case 13:
rate = 7200;
break;
case 14:
rate = 9600;
break;
case 15:
rate = 19200;
break;
}
setBaudRate(rate);
// Mask the lower four bits to get the baud rate.
int baudSelector = data & 0x0f;
switch (baudSelector) {
case 0:
rate = 0;
break;
case 1:
rate = 50;
break;
case 2:
rate = 75;
break;
case 3:
rate = 110; // Real rate is actually 109.92
break;
case 4:
rate = 135; // Real rate is actually 134.58
break;
case 5:
rate = 150;
break;
case 6:
rate = 300;
break;
case 7:
rate = 600;
break;
case 8:
rate = 1200;
break;
case 9:
rate = 1800;
break;
case 10:
rate = 2400;
break;
case 11:
rate = 3600;
break;
case 12:
rate = 4800;
break;
case 13:
rate = 7200;
break;
case 14:
rate = 9600;
break;
case 15:
rate = 19200;
break;
}
setBaudRate(rate);
}