NorthStar 10-sector formats

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Description and common features

10-sector hard-sectored formats used by NorthStar NSDOS and CP/M systems (e.g. NorthStar Horizon and NorthStar Advantage).

Two variants are known:

Both formats have 35 tracks, and 10 hardware-defined sectors.

Things still unknown:

Single-density format

References: NorthStar MDS-A-D Manual (p. 33)

Encoding method

Frequency Modulation. Two transitions per bit-cell, one clock (always present), one data (present if data bit is a '1', otherwise skipped). Data rate unknown.

Track format

Each track consists of ten sectors, whose position is specified by hardware (hard-sector index pulses). The NorthStar hardware triggers off of the leading edge of an index pulse, then waits for 96 microseconds to pass before enabling the read/write circuitry (this is the post index gap).

Each sector contains, in order:

Check character computation

From the NorthStar MDS-A-D Manual:

The check character is computed iteratively by setting it to zero and then exclusive ORing each successive data byte value with the current value of the check character and left cycling the result.

This source code (from Barry Watzman's disassembly of NS-DOS) makes it a little more obvious what's really happening:

READ	MOV	B,C		;MOVE 0 TO B
READ1	LDAX	D		;THIS IS THE ACTUAL DATA READ
	MOV	M,A		;STORE DATA IN MEMORY
	XRA	B		;I THINK THIS ADDS TO CKSUM
	RLC			;THIS TOO
	MOV	B,A		;MOVE PARTIAL CKSUM TO B
	INX	 H		;SET H FOR NEXT BYTE
	DCR	C		;GOING TO READ 256 BYTES & CKSUM
	JNZ	READ1		;READ MORE DATA
	LDAX	D		;READ THE CHECKSUM FROM DISK
	XRA	B		;COMPARE IT WITH CALCULATED CKSUM
	JZ	LE9C4		;IF THEY ARE SAME GO ON

There's also a disassembly of the Northstar boot PROM: http://www.sol20.org/northstar/nsboot.asm. There are some other interesting Northstar files on the same site: http://www.sol20.org/programs.html#Northstar.

What we can take away from this is:

Thus the C implementation of this algorithm would be something along the lines of:

// UNTESTED code to calculate the cyclic check code for a Northstar disc sector
unsigned char northstar_crc(unsigned char last_crc, unsigned char data) {
  unsigned char crc = last_crc;
  crc += data;
  crc = ((crc << 1)&0xfe) | ((crc >> 7)&1);
  return crc;
}

Double-density format

References: NorthStar MDS-A-D Manual (p. 33)

Encoding method

Modified Frequency Modulation. Two transitions per bit-cell, one clock (only present if the previous and current data bits are both zero), one data (present if data bit is a '1', otherwise skipped). Data rate unknown.

Track format

Each track consists of ten sectors, whose position is specified by hardware (hard-sector index pulses). The NorthStar hardware triggers off of the leading edge of an index pulse, then waits for 96 microseconds to pass before enabling the read/write circuitry (this is the post index gap).

Each sector contains, in order:

The check character is computed as per the Single-Density NorthStar format.

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