One caveat is that if you write a value to a “non-existent” IO port, you may be able to read that value back off the bus, if you read it immediately. However, in singletasking mode this is not an issue the CPU has nothing better to do — so polling is a good thing, then. And as always you guessed it I hope you learned something. As always I suggest you print it and have it with you when you read any further. On the Primary bus, this would be port 0x1F6. Now let’s look at the CMP a little closer. Or are you trying to implement a mouse-pointer GUI in your emulator?

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For the CPU equal is the same as zero with conditionals and it will always set the zero flag when the result of a mathematical operation is zero. With ST, each disk drive was connected to a controller board by two cables — a data cable, and a command cable. Try not to send bytes to the same IO port twice in a row.

CMP IOPort Driver Drivers

Not a full idea, and I am open to suggestions. Sign up or log in Sign up using Google. Okay, what is Chapter 4 about? This instruction uses one operand, the memory location to loop to.


If I understand it correctly, in interactive mode you have still to do iolort 4x out from one of the table. The number of bits to shift can either be an immediate a constant value or the CL register.

Many drives require a little time to respond to a “select”, and push their status onto the bus. DX needs to be increased to write the actuall RGB values. If the value read is 0, the drive does not exist.

The instruction returns AX as quotient and DX as remainder.

A SATA device will report 0x3c, and 0xc3 instead. These instructions are slow, and decrease performance. It is always necessary to validate the LBAs that are passed into your driver, as truly belonging to the partition that is being accessed. This gives a total of M sectors, or Iopotr of addressable space.

W e now have been working in iopodt mode some time, and I can imagine you want to make some graphics too.

If it did, it is necessary to read the Regular Status Register once, to make the disk clear its interrupt flag. That works fine However, I want to make it interactive, i. On some drives it is necessary to “manually” flush the hardware write cache after every write command. Kris Kamweru 6 4.

There is a special IO port bit that allows a driver to select either drive as the target drive for each command byte. Which means that a drive select may always happen just before a status read. Note for polling PIO drivers: After a CMP conditional instructions can be used to act on the result of the compare. Assume you have a sectorcount byte and a 28 bit LBA value.


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Retrieved from ” https: There is only one wire dedicated to selecting which drive on each bus is active. A conditional jump works like a normal jump, with the only difference that it jumps on a specific condition.

On the Primary bus, this would be port 0x1F6. W e’ve already seen the DIV instruction, but there are much more mathematical instructions.

Finally we output all the colours wich DS: If you are using polling see belowyou should account for the fact that your first four reads of the Status Register, after sending your command byte, may have the ERR or DF bits still set accidentally.