ATA PIO Mode: Difference between revisions
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m Updated 400ns delays section according to information from https://forum.osdev.org/viewtopic.php?p=317150#p317150 |
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The method suggested in the ATA specs for sending ATA commands tells you to check the BSY and DRQ bits before trying to send a command. This means that you need to read a Status Register (Alternate Status is a good choice) <i>for the proper drive</i> before sending the next command. Which means that you need to select the correct device <i>first</i>, before you can read that status (and then send all the other values to the other IO ports). |
The method suggested in the ATA specs for sending ATA commands tells you to check the BSY and DRQ bits before trying to send a command. This means that you need to read a Status Register (Alternate Status is a good choice) <i>for the proper drive</i> before sending the next command. Which means that you need to select the correct device <i>first</i>, before you can read that status (and then send all the other values to the other IO ports). |
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Which means that a drive select may always happen <i>just before</i> a status read. This is bad. Many drives require a little time to respond to a "select", and push their status onto the bus. The suggestion is to read the Status register <b> |
Which means that a drive select may always happen <i>just before</i> a status read. This is bad. Many drives require a little time to respond to a "select", and push their status onto the bus. The suggestion is to read the Status register <b>FIFTEEN TIMES</b>, and only pay attention to the value returned by the last one -- after selecting a new master or slave device. The point being that you can assume an IO port read takes at least 30ns, so doing the first fourteen creates a 420ns delay -- which allows the drive time to push the correct voltages onto the bus. |
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Reading IO ports to create delays wastes a lot of CPU cycles. So, it is actually smarter to have your driver remember the last value sent to each Drive Select IO port, to avoid doing unneeded drive selections, if the value did not change. If you do not send a drive select, then you only have to read the Status Register <b>once</b>. |
Reading IO ports to create delays wastes a lot of CPU cycles. So, it is actually smarter to have your driver remember the last value sent to each Drive Select IO port, to avoid doing unneeded drive selections, if the value did not change. If you do not send a drive select, then you only have to read the Status Register <b>once</b>. |
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