| Writing FCode 3.x Programs |    
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| Writing FCode 3.x Programs |
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| C H A P T E R 8 |
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Block and Byte Devices |
This chapter covers the following topics:
Block devices are nonvolatile mass storage devices whose information can be accessed in any order. Hard disks, floppy disks, and CD-ROMs are examples of block devices. OpenBoot typically uses block devices for booting.
This device type generally applies to disk devices, but as far as OpenBoot is concerned, it simply means that the device "looks like a disk" at the OpenBoot software interface level.
The block device's FCode must declare the block device type and implement the methods open and close, as well as the methods described in Required Methods.
Although packages of the block device type present a byte-oriented interface to the rest of the system, the associated hardware devices are usually block-oriented, that is, the device reads and writes data in blocks (groups of, for example, 512 or 2048 bytes). The standard /deblocker support package assists in the presentation of a byte-oriented interface overlaying a block-oriented interface, implementing a buffering layer that hides the underlying block length.
Block devices are often subdivided into several logical partitions as defined by a disk label--a special block, usually the first one on the device, which contains information about the device. The driver is responsible for appropriately interpreting a disk label. The driver may use the standard /disk-label support package if the device does not implement a specialized label. The /disk-label support package interprets a system-dependent label format. Since the disk booting protocol usually depends on the label format, the standard /disk-label support package also implements a load method for the corresponding boot protocol.
Byte devices are sequential-access mass storage devices, for example tape devices. OpenBoot typically uses byte and block devices for booting.
The byte device's FCode program must declare the byte device type and implement the open and close methods in addition to those described in Required Methods.
Although packages of the byte device type present a byte-oriented interface to the rest of the system, the associated hardware devices are usually record-oriented; the device reads and writes data in records containing more than one byte. The records may be fixed or variable length. The standard /deblocker support package assists in presenting a byte-oriented interface overlaying a record-oriented interface, implementing a buffering layer that hides the underlying record structure.
This method returns the record size block-len (in bytes) of all data transfers to or from the device. The most common value for block-len is 512.
This method is only required if the /deblocker support package is used.
load works differently for block and byte devices.
With block devices, it loads a standalone program from the device into memory at addr. size is the size in bytes of the program loaded. If the device can contain several such programs, the instance arguments returned by my-args can be used to select the program desired. open is executed before load is invoked.
With byte devices, load reads a standalone program from the tape file specified by the value of the argument string given by my-args. That value is the string representation of a decimal integer. If the argument string is null, tape file 0 is used. load places the program in memory at addr, returning the size of the read-in program in bytes.
This method returns the size in bytes of the largest single transfer that the device can perform. max-transfer is expected to be a multiple of block-size.
This method is only required if the /deblocker support package is used.
This method reads at most len bytes from the device into memory at addr. actual is the number of bytes read. If the number of bytes read is 0 or negative, the read failed. Note that len need not be a multiple of the device's normal block size.
This method reads #blocks records of length block-size bytes each from the device, starting at device block block#, into memory at address addr. #read is the number of blocks actually read.
This method is only required if the /deblocker support package is used.
seek works differently for block and byte devices.
For block devices, seek sets the device position for the next read or write. The position is the byte offset from the beginning of the device specified by the 64-bit number which is the concatenation of poshigh and poslow. status is -1 if the seek fails and 0 or 1 if it succeeds.
For byte devices, the method seeks to the byte offset in file file#. If offset and file# are both 0, rewind the tape. error? is -1 if seek fails and 0 if seek succeeds.
This method writes len bytes from memory at addr to the device. actual is the number of bytes written. If actual is less than len, the write did not succeed. len need not be a multiple of the device's normal block size.
This method writes #blocks records of length block-size bytes each to the device, starting at block block#, from memory at addr. #written is the number of blocks actually written.
If the device is not capable of random access (for example, a sequential access tape device), block# is ignored.
This method is only required if the /deblocker support package is used.
The structure of the device tree for the sample card supported by the sample device drivers in this chapter is as shown in FIGURE 8-1:
| Writing FCode 3.x Programs | 806-1379-10 |
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