[译文]硬盘指南：伺服格式化

会员3847

HARD DISK DRIVE GUIDE
How a Hard Disk Drive Works
Servo-Formatting
硬盘指南：硬盘是如何工作的---伺服格式化
Last updated: 2/5/2002

SERVO-FORMATTING.  Try to visualize a thing, hollow cylinder passing through all of the platters in a hard disk drive.  It would produce a circular track on each side of each platter.  Now divide each track into equal arcs or sectors.  Well, that is exactly how a hard disk is organized.  That is, Cylinders, Heads (which are equal the number of tracks/cylinder or platter sides), and Sectors are the coordinates of the data on a hard disk drive.
伺服格式化  让我们试着显示一下柱道是如何分布在硬盘里的盘片上的。在盘片的每一面生成着圆形的磁道，每个被划分为相等长度的弧或扇区。好吧，硬盘完全就是这样组织的---柱道，磁头（等于磁道/柱道数或盘片的面数），扇区是硬盘上放置数据的。

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There are two kinds of sectors on a hard disk.  The first and at the very lowest level is the servo sector.  When a hard disk is manufactured a special pattern is written in a code called a Gray code on the surface of the platters, while the drive is open in a clean room, with an expensive machine called a servowriter.
硬盘上有两类扇区，第1类处于最低级别的是伺服扇区。硬盘制造的时候，磁盘在洁净室里打开，用一种叫做伺服写入机的昂贵机器在盘片写一种叫做灰码的专门格式码。

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A Gray Code is a binary code in which successive numbers differ by only a single bit.  Although many Gray Codes are possible, one specific Gray Code is considered the Gray Code because of its efficiency in computation.  This efficiency is why it is used for the servo pattern instead of other binary codes.  A three-bit Gray Code is shown to the right.
灰码是一种二进制代码，相继的数字只差一个位。虽然有很多灰码可以使用，由于考虑到计算效率，选择这样一种特定的代码。

Although there are other schemes,  the Gray Code is written in a wedge at the start of each sector (an embedded servo pattern) on most drives.  There are a fixed number of servo sectors per track and the sectors are adjacent to one another.  This pattern is permanent and cannot be changed by writing normal data to the drive. It also cannot be changed by low-level formatting (see below) the drive, as some may think.  If it is changed, the drive has had it--kaput!
虽然还有其他方案，大多数硬盘上灰码以嵌入方式写到每个扇区的开头（嵌入伺服型）。每个磁道上有固定数目的伺服扇区与扇区互相邻接。这种代码是永久性的并且不能由在盘片上写普通数据来改变，也不能象某些人所想的那样由低格硬盘来改变。如果灰码改变，硬盘就完蛋了！

The electronics use feedback from the heads, which read the Gray code pattern, to very accurately position, and constantly correct the radial position of the appropriate head over the desired track, at the beginning of each sector, to compensate for variations in platter geometry, caused by mechanical stress and thermo expansion and contraction.   Altogether, the head positioning components form what is know as closed-loop servo system--a marvelous (and, perhaps, dangerous) thing to watch operate in a drive which has been opened.
磁头在每个扇区的头部读出灰码，利用灰码移到非常精确的位置，不断校正磁头本身的位置，使得能处于想要到达的磁道，补偿由机械应力和热胀冷缩引起的盘片几何改变。总之，已知磁头定位系统是一个闭环伺服系统---观察打开的硬盘工作是一件很爽的（也许是很危险的）事情。

会员13276

真是佩服老张.能够找到这么多好资料.先收下啦!

会员3847

PS: 据我所知，在完全RAW的盘面上写灰码是由伺服写入机控制和定位硬盘本身所带的磁头写的。

会员42712

哇太深了有点看不懂

会员28226

暂时还理解不了!感觉像VCD的轨道方式,但硬盘可以自校,灰码是不是重新生成呢!

会员34899

张先生的英文这么好，真是佩服得不得了！

会员97

SF也是依据伺服信息（灰度）来完成的，现在市面上据说有灰度码写入机，有些机构在使用了，好像价格不便宜

会员55141

张先生太好了！能学到很多东西！这些原来可是没听说过！

会员3847

原帖由 华盛 于 2008-5-3 23:51 发表

                               
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暂时还理解不了!感觉像VCD的轨道方式,但硬盘可以自校,灰码是不是重新生成呢!

原则上灰码是不能在普通条件下生成的。但据说有一个可以探索的方法：就是利用其中一个盘面的灰码，通过特殊操作生成其他盘面的灰码。

会员1641

原帖由 张先生 于 2008-5-24 08:26 发表

                               
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原则上灰码是不能在普通条件下生成的。但据说有一个可以探索的方法：就是利用其中一个盘面的灰码，通过特殊操作生成其他盘面的灰码。

这个"据说"很有点意思.张先生能透露一下消息来源吗?希望能做更深一步地探究.

会员32347

能学到很多东西！这些原来可是没听说过！

会员61373

谢谢张老师，好东西，收藏了