Caple C871i Manual do Utilizador descarregar pdf (Página 146)

Apple II Computer Info

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### Created : Friday, September 27, 1996 Modified: Friday, September 27, 1996

### File Type: "TEXT" File Creator: "LMAN"

### File Size: 4731 bytes 4 KB

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state.edu!howland.reston.ans.net!nntp.coast.net!news.kei.com!newsfeed.internetmci.

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From: [email protected] (Neil Parker)

Newsgroups: comp.sys.apple2.programmer

Subject: Re: Q: 3.5" encoding scheme

Date: 24 Jun 1996 09:50:51 GMT

Organization: University of Oregon Campus Information Exchange

Lines: 98

Distribution: world

Message-ID: <[email protected]>

References: <[email protected]>

NNTP-Posting-Host: cie-2.uoregon.edu

In article <[email protected]> [email protected] (Henrik

'Ratte' Gudat) writes:

>[...snip...]

>- as with 5.25" disks, address fields are encoded 4&4, the data chunk 6&2,

>right?

Wrong. The address fields of a 3.5-inch disk are encoded 6&2 (sort of).

The address field looks like this:

$D5, $AA, $96, track, sector, sides, format, checksum, $DE, $AA

Each datum in the address field is a 6-bit number. Their meanings are as

follows:

track The low-order six bits of the track number

sector The sector number

sides Bit 0 is the high-order bit of the track number

Bit 5 is the side number (0 for side 1, 1 for side 2)

All other bits are zero (i.e. the number looks like "s0000t")

format The low four bits are the interleave (usually 2, unless the

disk was formatted on a Unidisk 3.5)

The high bit is the number of sides (0=single, 1=double)

Bit 4 is always 0 (the number looks like "s0iiii")

checksum A checksum of the previous four values (I'm not sure, but

I think it's computed by EORing)

Each of these numbers is translated to a valid disk byte by looking it up in

the 6&2 translation table (so that 0 becomes $96, 1 becomes $97, 2 becomes

$9A, etc.).

>- the two bits that are rolled out of each byte (6&2 scheme) are put into a

>secondary buffer. Where will this buffer be written? After the main buffer

>(with bits 2-8 of each byte)? or will it also be divided by some magic figure

>(for example, 6 chunks of 86 bytes from main buffer and 27 bytes from secondary

>each)?

Apple II Computer Technical Information : Apple II Family Hardware Info

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