LSX - Revision history

Jnc: /* External links */ complete cats

2022-11-08T06:14:34Z

‎External links: complete cats

Jnc: /* External links */ +UNIX on a Microprocessor

2022-09-29T19:58:47Z

‎External links: +UNIX on a Microprocessor

Jnc: /* Modified commands */ 'mkfs' too

2022-08-01T11:55:24Z

‎Modified commands: 'mkfs' too

Jnc: /* Implementation details */ max of 100 inodes

2022-08-01T11:52:54Z

‎Implementation details: max of 100 inodes

Jnc: /* Further reading */ +link to online copy

2022-01-06T22:49:44Z

‎Further reading: +link to online copy

Jnc: tweaks

2020-12-26T18:26:20Z

tweaks

Jnc: /* Implementation details */ details of contiguous

2020-12-20T14:58:02Z

‎Implementation details: details of contiguous

Jnc: /* Further reading */ links to repos

2020-12-19T17:59:53Z

‎Further reading: links to repos

Jnc: More details; mostly from the source

2020-12-19T10:19:53Z

More details; mostly from the source

Jnc: Basically done

2020-12-19T03:00:46Z

Basically done

New page

'''LSX''' is a somewhat limited variant of [[UNIX Sixth Edition|V6 Unix]], for use on the [[LSI-11]] model of the [[PDP-11], which did not have [[memory management]] (which was required for early Unix versions from 4 onward). It was targeted to systems with as little as 40KB of main memory. Although it is not full Unix, it retains a large amount of the functionality of the original.

Most V6 [[application]]s run without [[source code|source]] modification on LSX, although it is not [[object code|binary]]-compatible with V6. This is a result of the lack of memory management on LSI-11; the [[kernel]] and [[user]] commands (as [[process]]es) share a single PDP-11 [[address space]], with the kernel in low memory, and the processes (one at a time) in high memory.

There was a second, later, stripped-down variant of V6 for PDP-11's without memory management, [[MINI-UNIX]]; it differed from LSX in being targeted at machines less limited amounts of [[main memory]] (a real concern for LSX, on early LSI-11s, which usually came with very limited amounts).

==Implementation details==

Because of the way user commands are run in high memory in a single address space, user commands are [[relocatable binary|linked]] to run at a high [[address]] (usually 040000, instead of at 0 as on standard V6.

Thus, 16KB (notionally) is normally allocated to hold the kernel (with a small number of processes and [[disk]] [[buffer]]s), leaving up to 40KB (the exact amount will depend on the amount of memory available on the machine) available for user commands/applications. (The "notionally" is because that 16KB also includes the 'user' structure: a per-process [[swapping|swappable]] data area, 1KB in size; thus 15KB is actually available for the kernel.)

This division can be altered, albeit painfully (all the command binaries have to be re-linked to the new address), to allow the size of the kernel to vary.

The interface to the I/O system is somewhat the same as that of V6, so that existing V6 [[device driver]]s can easily be modified to run under LSX. In particular, there can only be one type each of 'character' (un-structured) devices and 'block' ([[mass storage]]) devices (unlike V6, which allows a semi-arbitrary number of each); the single character device ''must'' be a 'tty' device. Other than that, the interface to character devices is identical, as is that to the tty driver; the interface to the block device buffer system is also identical.

Two other minor aspects that are changed are:

* since the system is limited to less than 64KB of memory, the 'extended memory' field in the buffer header is not present;
* 'raw devices' (which can do [[Direct Memory Access|DMA]] transfers directly from the device to a buffer in the user's process) are not supported.

The [[file system]] apparently supports contiguous storage of files, as an alternative to the normal random allocation of blocks in UNIX;; the details are currently obscure. This was probably done to produce higher performance with the [[floppy disk]]s intended for mass storage on LSI-11 LSX systems. 'Read-ahead' has also been removed from the file system implementation; this saves on address space in the kernel at the cost of a slight degradation of performance.

The [[scheduler]] is a simple round-robin time-slicing one.

LSX uses the standard [[C programming language|C]] [[compiler]], which generate [[PDP-11 Extended Instruction Set|EIS]] intructions. For LSI-11s which do not have the [[KEV11-A floating point|KEV11-A]] or -B chip to provide EIS, an [[simulation]] package is available which is called into action when a [[trap]] is produced by an otherwise un-supported [[instruction]].

===User-visible compatability===

MINI-UNIX supports almost all the standard V6 system calls, with the excepion of:

* pipe()
* ptrace()
* getid()/setgid()
* prof()

[[Pipe]]s not supported not only because of space in the OS, but because a process swap-out and -in will be required every time a different stage needs to run, leading to poor performance.

Although pipes are not supported, the [[command processor|shell]] supports the pipe command syntax ('|'), by use of temporary files (modulo the amount of free space on the disk, of course).

The removal of ptrace() means there is no interactive [[debug]]ging; this is not a major hardship, as the lack of memory management means a buggy user program can crash the system anyway.

Although the system retains a limited capability to join a separate filesystem on a second mass storage device onto the file system hierarchy, the capability is not general; the user cannot 'mount' and 'unmount' file systems.

There is no support for command images with 'pure texts', or for so-called 'split I+D' mode.

The maximum file size supported is 1MB; 'large' files are supported, but not 'huge' files (those with two levels of indirect blocks).

===Modified commands===

Only a few commands have significant changes:

* Shell - no pipes
* Linker 'ld' - minor modification to change target address
* [[Debugger]] 'db' - minor modification to change program address

A few other commands have minor changes, but they are not of any great consequence. (See Section 4 of the "The MINI-UNIX System" memorandum for details.) Some have been slightly modified to fit into the 32KB available for user processes.

==Further reading==

* Heinz Lycklama, ''UNIX on a Microprocessor'', Bell System Technical Journal, Vol. 57, No. 6, July-August 1978



[[Category: Unix OS's]]

← Older revision		Revision as of 06:14, 8 November 2022
Line 79:		Line 79:
	* [https://www.tuhs.org/cgi-bin/utree.pl?file=LSX LSX] - LSX at the [[The Unix Heritage Society\|TUHS]] archive		* [https://www.tuhs.org/cgi-bin/utree.pl?file=LSX LSX] - LSX at the [[The Unix Heritage Society\|TUHS]] archive

		+	{{Nav Unix}}
		+
		+	[[Category: PDP-11 Operating Systems]]
		+	[[Category: Non-DEC Operating Systems]]
	[[Category: Unix OS's]]		[[Category: Unix OS's]]

@@ Line 75: / Line 75: @@
 ==External links==
 * [http://www.mailcom.com/lsx/index.html LSX Unix Restoration Page]
 * [https://www.tuhs.org/cgi-bin/utree.pl?file=LSX LSX] - LSX at the [[The Unix Heritage Society|TUHS]] archive
 [[Category: Unix OS's]]

@@ Line 13: / Line 13: @@
 This division can be altered, albeit painfully (all the command binaries have to be re-linked to the new address), to allow the size of the kernel to vary.
-The interface to the I/O system is somewhat the same as that of V6, so that existing V6 [[device driver]]s can easily be modified to run under LSX. In particular, there can only be one type each of 'character' (un-structured) devices and 'block' ([[mass storage]]) devices (unlike V6, which allows a semi-arbitrary number of each); the single character device ''must'' be a 'tty' ([[terminal]]) device. Other than that, the interface to character devices is identical, as is that to the tty driver; the interface to the block device buffer system is also identical.
+The interface to the I/O system is somewhat the same as that of V6, so that existing V6 [[device driver]]s can easily be modified to run under LSX. In particular, there can only be one type each of 'character' (un-structured) devices and 'block' ([[mass storage]]) devices (unlike V6 and MINI-UNIX, which allow a semi-arbitrary number of each); the single character device ''must'' be a 'tty' ([[terminal]]) device. Other than that, the interface to character devices is identical, as is that to the tty driver; the interface to the block device buffer system is also identical.
 Two other minor aspects that are changed are:
@@ Line 20: / Line 20: @@
 * 'raw devices' (which can do [[Direct Memory Access|DMA]] transfers directly from the device to a buffer in the user's process) are not supported.
-The [[file system]] supports contiguous storage of files, as an alternative to the normal random allocation of blocks in UNIX. This was probably done to produce higher performance with the [[floppy disk]]s intended for mass storage on LSI-11 LSX systems. The details are currently obscure, since the [[kernel]] does not support the creation of such files; the paper describing LSX (below) indicates there were two separate programs, one to allocate space for such files, and one to move a file into such an area, but they do not seem to be extant. When contiguous files are in use, the free list is kept as an array in the superblock, rather than a threaded list (the normal method in V6).
+The [[file system]] supports contiguous storage of files, as an alternative to the normal random allocation of blocks in UNIX. This was probably done to produce higher performance with the [[floppy disk]]s intended for mass storage on LSI-11 LSX systems. The details are currently obscure, since the [[kernel]] does not support the creation of such files; the paper describing LSX (below) indicates there were two separate programs, one to allocate space for such files, and one to move a file into such an area, but they do not seem to be extant. When contiguous files are in use, the free list is kept as an array in the superblock, rather than as a threaded list of blocks (the normal method in V6).
 'Read-ahead' has also been removed from the file system implementation; this saves on address space in the kernel at the cost of a slight degradation of performance.

@@ Line 20: / Line 20: @@
 * 'raw devices' (which can do [[Direct Memory Access|DMA]] transfers directly from the device to a buffer in the user's process) are not supported.
-The [[file system]] apparently supports contiguous storage of files, as an alternative to the normal random allocation of blocks in UNIX;; the details are currently obscure. This was probably done to produce higher performance with the [[floppy disk]]s intended for mass storage on LSI-11 LSX systems. 'Read-ahead' has also been removed from the file system implementation; this saves on address space in the kernel at the cost of a slight degradation of performance.
+The [[file system]] supports contiguous storage of files, as an alternative to the normal random allocation of blocks in UNIX. This was probably done to produce higher performance with the [[floppy disk]]s intended for mass storage on LSI-11 LSX systems. The details are currently obscure, since the [[kernel]] does not support the creation of such files; the paper describing LSX (below) indicates there were two separate programs, one to allocate space for such files, and one to move a file into such an area, but they do not seem to be extant. When contiguous files are in use, the free list is kept as an array in the superblock, rather than a threaded list (the normal method in V6).
 The [[scheduler]] is a simple round-robin time-slicing one. The system optionally supports a 'background' [[process]], which runs when foreground processes are waiting for tty input.
@@ Line 28: / Line 30: @@
 ===User-visible compatability===
-LSX supports almost all the standard V6 system calls, with the excepion of:
+LSX supports most of the standard V6 system calls, with the excepion of:
 * pipe()
 * ptrace()
 * getid()/setgid()
 * prof()
 * mount()
 * umount()
 * chown()
 [[Pipe]]s not supported not only because of space in the OS, but because a process swap-out and -in will be required every time a different stage needs to run, leading to poor performance.

@@ Line 65: / Line 65: @@
 * Linker 'ld' - minor modification to change target address
 * [[Debugger]] 'db' - minor modification to change program address
 A few other commands have minor changes, but they are not of any great consequence. Some have been slightly modified to fit into the 32KB available for user processes.

@@ Line 56: / Line 56: @@
 There is no support for command images with 'pure texts', or for so-called 'split I+D' mode.
-The maximum file size supported is 1MB; 'large' files are supported, but not 'huge' files (those with two levels of indirect blocks).
+The maximum file size supported is 1MB; 'large' files are supported, but not 'huge' files (those with two levels of indirect blocks). LSX also does not support file systems with more than 100 [[inode]]s; again, almost certainly bringing a slight reduction in kernel size.
 ===Modified commands===

@@ Line 70: / Line 70: @@
 ==Further reading==
-* Heinz Lycklama, ''UNIX on a Microprocessor'', Bell System Technical Journal, Vol. 57, No. 6, July-August 1978
+* Heinz Lycklama, [https://www.heinzlycklama.com/docs/bstj57-6-2087.pdf. ''UNIX on a Microprocessor''], Bell System Technical Journal, Vol. 57, No. 6, July-August 1978
 ==External links==

@@ Line 1: / Line 1: @@
-'''LSX''' is a somewhat limited variant of [[UNIX Sixth Edition|V6 Unix]], for use on the [[LSI-11]] model of the [[PDP-11], which did not have [[memory management]] (which was required for early Unix versions from 4 onward). It was targeted to systems with as little as 40KB of main memory. Although it is not full Unix, it retains a large amount of the functionality of the original.
+'''LSX''' is a somewhat limited variant of [[UNIX Sixth Edition|V6 Unix]], intended for use on the [[LSI-11]] model of the [[PDP-11]], which did not have [[memory management]] (which was required for early Unix versions from 4 onward). Support for other models without memory management (such as the [[PDP-11/05]]) was also included. It was targeted to systems with a minimal amount of [[main memory]], as little as 40KB (a real concern for LSX, on early LSI-11s, which usually came with very limited amounts). Although it is not full Unix, it retains a large amount of the functionality of the original.
 Most V6 [[application]]s run without [[source code|source]] modification on LSX, although it is not [[object code|binary]]-compatible with V6. This is a result of the lack of memory management on LSI-11; the [[kernel]] and [[user]] commands (as [[process]]es) share a single PDP-11 [[address space]], with the kernel in low memory, and the processes (one at a time) in high memory.
-There was a second, later, stripped-down variant of V6 for PDP-11's without memory management, [[MINI-UNIX]]; it differed from LSX in being targeted at machines less limited amounts of [[main memory]] (a real concern for LSX, on early LSI-11s, which usually came with very limited amounts).
+There was a second, later, stripped-down variant of V6 for PDP-11's without memory management, [[MINI-UNIX]]. It differed from LSX in being for machines less limited amounts of main memory; it was targeted at machines with the full 56KB possible on such PDP-11's.
 ==Implementation details==
@@ Line 9: / Line 9: @@
 Because of the way user commands are run in high memory in a single address space, user commands are [[relocatable binary|linked]] to run at a high [[address]] (usually 040000, instead of at 0 as on standard V6.
-Thus, 16KB (notionally) is normally allocated to hold the kernel (with a small number of processes and [[disk]] [[buffer]]s), leaving up to 40KB (the exact amount will depend on the amount of memory available on the machine) available for user commands/applications. (The "notionally" is because that 16KB also includes the 'user' structure: a per-process [[swapping|swappable]] data area, 1KB in size; thus 15KB is actually available for the kernel.)
+Thus, 16KB (notionally) is normally allocated to hold the kernel (with a small number of processes and [[disk]] [[buffer]]s), leaving up to 40KB (the exact amount will depend on the amount of main memory on the machine) available for user commands/applications. (The "notionally" is because that 16KB also includes the 'user' structure: a per-process [[swapping|swappable]] data area, 1KB in size; thus 15KB is actually available for the kernel.)
 This division can be altered, albeit painfully (all the command binaries have to be re-linked to the new address), to allow the size of the kernel to vary.
-The interface to the I/O system is somewhat the same as that of V6, so that existing V6 [[device driver]]s can easily be modified to run under LSX. In particular, there can only be one type each of 'character' (un-structured) devices and 'block' ([[mass storage]]) devices (unlike V6, which allows a semi-arbitrary number of each); the single character device ''must'' be a 'tty' device. Other than that, the interface to character devices is identical, as is that to the tty driver; the interface to the block device buffer system is also identical.
+The interface to the I/O system is somewhat the same as that of V6, so that existing V6 [[device driver]]s can easily be modified to run under LSX. In particular, there can only be one type each of 'character' (un-structured) devices and 'block' ([[mass storage]]) devices (unlike V6, which allows a semi-arbitrary number of each); the single character device ''must'' be a 'tty' ([[terminal]]) device. Other than that, the interface to character devices is identical, as is that to the tty driver; the interface to the block device buffer system is also identical.
 Two other minor aspects that are changed are:
@@ Line 22: / Line 22: @@
 The [[file system]] apparently supports contiguous storage of files, as an alternative to the normal random allocation of blocks in UNIX;; the details are currently obscure. This was probably done to produce higher performance with the [[floppy disk]]s intended for mass storage on LSI-11 LSX systems. 'Read-ahead' has also been removed from the file system implementation; this saves on address space in the kernel at the cost of a slight degradation of performance.
-The [[scheduler]] is a simple round-robin time-slicing one.
+The [[scheduler]] is a simple round-robin time-slicing one. The system optionally supports a 'background' [[process]], which runs when foreground processes are waiting for tty input.
-LSX uses the standard [[C programming language|C]] [[compiler]], which generate [[PDP-11 Extended Instruction Set|EIS]] intructions. For LSI-11s which do not have the [[KEV11-A floating point|KEV11-A]] or -B chip to provide EIS, an [[simulation]] package is available which is called into action when a [[trap]] is produced by an otherwise un-supported [[instruction]].
+LSX uses the standard [[C programming language|C]] [[compiler]], which generate [[PDP-11 Extended Instruction Set|EIS]] intructions. For LSI-11s which do not have the [[KEV11-A floating point|KEV11-A]] or -B chip to provide EIS, and other PDP-11 models which do not have EIS, an [[simulation]] package is available which is called into action when a [[trap]] is produced by an otherwise un-supported [[instruction]].
 ===User-visible compatability===
-MINI-UNIX supports almost all the standard V6 system calls, with the excepion of:
+LSX supports almost all the standard V6 system calls, with the excepion of:
 * pipe()
@@ Line 34: / Line 34: @@
 * getid()/setgid()
 * prof()
 [[Pipe]]s not supported not only because of space in the OS, but because a process swap-out and -in will be required every time a different stage needs to run, leading to poor performance.
-Although pipes are not supported, the [[command processor|shell]] supports the pipe command syntax ('|'), by use of temporary files (modulo the amount of free space on the disk, of course).
+Although pipes are not supported, the [[command processor|shell]] supports the pipe command syntax ('|'), by use of temporary [[file]]s (modulo the amount of free space on the disk, of course).
 The removal of ptrace() means there is no interactive [[debug]]ging; this is not a major hardship, as the lack of memory management means a buggy user program can crash the system anyway.
 Although the system retains a limited capability to join a separate filesystem on a second mass storage device onto the file system hierarchy, the capability is not general; the user cannot 'mount' and 'unmount' file systems.
 There is no support for command images with 'pure texts', or for so-called 'split I+D' mode.
@@ Line 55: / Line 60: @@
 * [[Debugger]] 'db' - minor modification to change program address
-A few other commands have minor changes, but they are not of any great consequence. (See Section 4 of the "The MINI-UNIX System" memorandum for details.) Some have been slightly modified to fit into the 32KB available for user processes.
+A few other commands have minor changes, but they are not of any great consequence. Some have been slightly modified to fit into the 32KB available for user processes.
 ==Further reading==