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IDEDOS: Difference between revisions
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→8-bit vs 16-bit interfaces for Z80 based computers
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Since the [[Z80]] microprocessor has as an 8-bit data bus and IDE requires a 16-bit data bus some circuitry is required to combine a pair of 8-bit read/write operations into a single 16-bit operation. This extra circuitry increases cost and complexity in a hard disk interface so some interfaces omit this circuitry and simply treat the IDE bus as a normal 8-bit wide I/O device. This means that when any 16-bit word is read from or written to the hard disk, the upper byte is simply ignored, therefore half the hard disk's capacity cannot be utilised by computer. | Since the [[Z80]] microprocessor has as an 8-bit data bus and IDE requires a 16-bit data bus some circuitry is required to combine a pair of 8-bit read/write operations into a single 16-bit operation. This extra circuitry increases cost and complexity in a hard disk interface so some interfaces omit this circuitry and simply treat the IDE bus as a normal 8-bit wide I/O device. This means that when any 16-bit word is read from or written to the hard disk, the upper byte is simply ignored, therefore half the hard disk's capacity cannot be utilised by computer. | ||
IDEDOS supports these halved capacity interfaces as it simply means that each hard disk sector contains 256 bytes instead of the standard 512 bytes. Software which reads from an IDEDOS formatted disk should not therefore make assumptions about the number of bytes per sector as it depends on the hardware used to format the disk. | IDEDOS supports these halved capacity interfaces as it simply means that each hard disk sector contains 256 bytes instead of the standard 512 bytes. Software which reads from an IDEDOS formatted disk should not therefore make assumptions about the number of bytes per sector as it depends on the hardware used to format the disk. | ||
A Logical Sector is defined as 512 bytes therefore the number of logical sectors on an 8-bit formatted drive will be half the number of physical sectors and the offset calculated accordingly. | |||
====The IDEDOS System Partition==== | ====The IDEDOS System Partition==== | ||
