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Inter-Office Memorandum
To Mesa Users Date May 31, 1978
From John Wick' Location Palo Alto
Subject Mesa 4.0 System Update Organization SDD/SD
XEROX
Filed on: [IRIS]
This memo outlines changes made in the Mesa system code since the last release (October 17,
1977). It also dicusses a number of internal changes made in the system and the microcode;
see also the Mesa 4.0 Microcode Update. (In addition, the list of change requests closed by
Mesa 4.0 will appear as part of the Software Release Description.)
External Interfaces
Names in square brackets refer to sections of the Mesa System Documentation which has
also been updated. More details can be found there and in other documentation
accompanying this release.
Alto Reserved Locations
Mesa software now conforms to the most recent allocation of Alto reserved locations (Alto:
A Personal Computer System, Hardware Manual, February, 1978, Appendix H). The only
page one location reserved by Mesa is the disaster flag (location 456B).
Basic Mesa
Basic Mesa has been reclassified as released software. To facilitate development of special
purpose systems, Basic Mesa no longer includes a keyboard handler; the procedure for
adding one is described in the documentation on the Keyboard Package. [Section 3]
CheckPoint/ Restart
Procedures have been added for writing checkpoint image files, and the bootstrap loader has
been modified to load them. Checkpoint files contain only the data (so creating them and
loading them is fast); unlike Makelmage, MakeCheckPoint does not copy code or any other
files to the image file. Note this means that none of the files referenced by the checkpoint
can be updated or modified in any .way. [Image Files]
Code Links
The loader has been extended to optionally write external links in the code rather than the
global frame (assuming code links were specified in the configuration description). To
effect this option, the loader II switch must be used. Note that if a module calls for code
links, loading it will be slower, as the code segment must be swapped in and rewritten. To
decrease resident storage requirements, all standard Mesa systems are configured with links
in the code. [Modules]
.Mesa 4.0 System Update 2
Convert
Due to extremely limited microcode space, the convert instruction is no longer part of the
Mesa instruction set. It can be simulated with BitBlt (see the system display package).
-[Display]
Debugger Call
A method of invoking the debugger explicitly, without generating a signal, is now available;
it causes minimal disruption of the current state of the debuggee. The inline CallDebugger
is defined in MiscDefs. [Miscellaneous]
Deleting Configurations
The procedure UnNewConfig is now available for unloading a configuration from a running
system. The configuration's global frames are deallocated and its code segments are released.
In addition, a'll existing modules are checked for bindings to the configuration being
unloaded. [Modules]
Display Package
A smaller display package, similar to the Bcpl version, is now standard. It supports a simple
display oriented teletype-like interface and an optional typescript file (see FontDefs and
DisplayDefs). The window package is available as a separate configuration (which is no
longer supported); see Window Package. [Display Package]
File Lengths
File lengths and file length hints are no longer kept as a permanent part of each file object.
A separate (and optional) length object is allocated only when the length of a file is
requested. Since length objects are not required for files containing code segments, this
substantially reduces the amount of resident object space required to handle a large number
of BeDS. [File Package]
Free Storage Package
The free storage package has been modified so that it protects each zone (including the
system supplied free storage heap) with a monitor. This enables several processes to share
the heap. [Storage Management]
Interrupts
For compatability with the new process mechanism, a Nova interrupt now causes a "naked
notify" to one of sixteen condition variables. Pointers to these condition variables are
contained in fixed locations in page zero (see ProcessDefs). [Processes and Monitors]
Images
All symbol table references and options have been removed from the system, and the
interface to Makelmage has been changed to reflect this (the symbolsTolmage parameter has
been dropped). The image file format has also been revised to support checkpoint/restart
files (see above). [Image Files]
KeyStreams
The keyboard routines have been revised to utilize the new process mechanism; a condition
variable is notified when characters are available in the current key stream. The "idle
procedure" has been replaced by a WAIT on this condition variable. [Keyboard Package]
.Mesa 4.0 System Update 3
Memory Management
A number of options have been added to the memory management and swapping facilities.
Unlocked read-only segments, such as fonts, are now swapped automatically (previously, this
applied only to code segments). The interface to swapping procedures has been expanded to
include an (optional) Alloclnfo parameter, which provides more information about how the
memory should be allocated. The new facilities are defined in AllocDefs; swap stratagies and
swapping procedures are now defined there. [Segment Package]
Pause to Debugger
A switch has been added to the NEW command which will invoke the debugger as soon as a
configuration has been loaded (in command line mode, before it is started). Also, holding
down the control-swat keys while an image file is loaded will now work correctly (the
debugger will be invoked as soon as possible). [Section 4]
Process Structure
A new process mechanism which supports monitors and condition variables, WAITS and
NOTIFYS, and FORK and JOIN has been implemented. The BLOCK operation has been eliminated
(a Yield procedure is available). Note that several refinements (and some revisions) of the
original proposal (in the Pilot Functional Specijication) have been made. A new chapter in
the Mesa Language Manual provides complete documentation; additional facilities which
are not part of the language are described in the system documention (and ProcessDefs).
[Processes and Monitors]
Warning: In general, facilities provided by the system are not protected by monitors. Since
Pilot will be available soon, we have not redesigned and retrofitted the Alto/Mesa system to
support preemptive processes (other than simple interrupt routines, as before). Except for
the free storage package (see above), system facilities shared by more than one process must
be protected by a user supplied set of monitor entry procedures.
Run Image
A module is now available which will invoke a Mesa image file (or a Bcpl run file) from the
Mesa environment, without returning to the Alto Executive. Any Mesa subsystem which
supports command line input (e.g., the compiler or binder, or even the Mesa system itself)
can be invoked considerably faster using this facility. [Image Files]
StreamIO
To make instantiating multiple instances of this module easier, StreamlO no longer takes
parameters specifying the input and output streams. Procedures are available which override
the default settings. [StreamIO Package]
StringDejs
To facilitate conversion from binary to alphanumeric data, AppendNumber and other related
procedures have been added to StringDefs. WordsForString now expects a cardinal. Bcpl
strings now use packed arrays. [String Package]
SystemDejs
A simplified interface to the new memory management facilities has been added in the form
of two additional procedures: AllocateResidentSegment and AllocateResidentPages. [Storage
Management]
TrapDejs
Documentation on a number of system generated traps is now available in a new section of
the system document. Most traps are converted into signals of the same name: StartFault,
ControlFault, UnboundProcedure, StackError, etc. [Traps]
.Mesa 4.0 System Update 4
UnNew
This procedure no longer supports the option of adding the module's frame to the free
frame heap (it will "be returned there only if it was allocated from there). Note that this
procedure does not check for other modules bound to the one being deleted. Beware of
dangling referencesl [Modules]
Unsigned Compare
The unsigned compare operation (usc) has been removed from InlineDefs. Use of the
appropriate signed or unsigned comparison operators should be controlled by the type of the
variables involved: CARDINAL (unsigned) or INTEGER (signed). See the Mesa Language Manual
and the Mesa 4.0 Compiler Update for more information.
User Interface
Modules to be loaded into the standard Mesa system (and Basic Mesa) can now be specified
on the command line (and in command files). A number of switches are available to
control loading options; these are described in the Mesa User's Handbook.
VMnotFree
The signal VMnotFree was inadvertently respelled (it used to be VMNotFree). [Segment
Package]
Window Package
The Mesa window package is no longer part of the standard Mesa system; it is available as a
separate configuration. WindEx replaces WManager for optional use in the debugger. The
definitions of BitBlt and Convert have been removed from RectangleDefs. Support for a
blinking cursor has been added. [Window Package]
Internal Interfaces
The following changes are internal to the implementation and do not affect public
interfaces. They may affect performance and/or space requirements, however. For several
of these items, furthur information can be founded in the Mesa 4.0 Microcode Update.
Alto/ Mesa Microcode
The microcode has been completely rewritten to improve its execution speed. The major
changes are: 1) several instructions must now be aligned on word boundaries and, 2) certain
instructions (notably jumps) require the evaluation stack to be empty except for their
operands. (As a side effect of the reorganization, new opcode numbers have been assigned
to most instructions.) We have observed improvements in raw execution speed of 20-50%,
depending on the dynamic instruction mix.
Alto File System
The hint in the DiskDescriptor containing the number of free disk pages is now maintained
properly. The declarations describing Sys.Log have been deleted, since it is no longer
supported by the Bcpt OS (versions 14 or later).
Alto Time Standard
The time conversion package is now part of standard system. UnpackedTime and PackDT
have been extended to support GMT, time zones, and daylight savings time (for
compatability with Hcpl OS versions 14 or greater).
.Mesa 4.0 System Update 5
Bcd Format
This structure has been revised to achieve a space reduction of about 10%. A segment table
and name table have been added, as well as support for packed code segments and code
links. Provision for a source version stamp has also been included.
BitBlt
BitBlts are now performed entirely in microcode, using the ROM subroutine. They are not
only faster, but interruptable as well. BitBltOefs now contains the interface to this
operation; it has been updated to include the extended memory option.
Cleanup Procedures
Adding a cleanup procedure must now specify the conditions under which the procedure
should be called. Several cleanup procedures are no longer called on swapping to and from
the debugger. :
Warning: Since the interrupt key may preempt a process holding a monitor lock. cleanup
procedures must not attempt to enter any monitor. This severly restricts the operations that
can be safely performed by cleanup procedures.
Code Packing
All resident code now employs the packed code option implemented by the binder.
Code Segments
The format of code segments has been revised to accomodate the new options for handling
external links (storing them in the code and storing them backwards from the frame or code
base). Also note that when several modules are packed into the same code segment, only the
LRU bit of the first module is examined by the swapper.
ControlDels
The declarations of control links and local and global frames now use overlaid variant
records. The AV. SO. and GFT have been preassigned constant values as in the PrincOps; the
REGISTER construct has been revised accordingly (see also Trap Parameters). The assignment
of System Data indicies is now contained in SOOefs.
Descriptor Instructions
The descriptor instructions (OESCB and DESCBS) described in the PrincOps have been
implemented.
External Links
External links are now stored and indexed backwards from the global frame base (or code
base); this eliminates the "effective" minimum frame size overhead of eighteen. The total
number of external procedures, programs, signals, and errors per module must be less than
256.
Field Descriptors
The format of field descriptors has been revised to agree with the DO design. The read field
stack (RFS) and read. field code (RFC) instructions have been implemented.
Frame Allocation
The ALLOC and FREE instructions are now implemented in microcode; thus the overhead for
large (greater than five word) parameter and result records has been drastically reduced.
,Mesa 4.0 System Update 6
Global Frame Format
The global frame overhead has been reduced from ten to three words; all fields relating to
the old binding scheme have been eliminated (see also External Links and Main Body
Procedure).
Kernel Function Calls
Several new kernel functions have been added as a result of other extensions (see SDDefs).
Most entries of public interest are now defined as inlines in definitions modules (e.g.
FrameDefs, LoaderDefs). Provision has been made for all of the traps defined in the
PrincOps.
Load State Format
A change in format has reduced the size of the load state substantially. The maximum
number of BCDS which can be loaded into a single image file is now about forty.
Long Integers
Addition, subtraction, and comparison of long (32-bit) integers are now implemented in
microcode. Multiplication and division are done by software (and are therefore slow).
Main Body Procedure
The main body of a module is now executed in a separate local frame, instead of using the
global frame. This eliminates three words from the global frame overhead (the access link,
saved pc, and return link).
Nil Pointers
To enable conversion and comparison of both long and short values of null pointers, the
val ue of NIL has been changed.
Novacode interface
To accomodate the implementation of the process opcodes in Nova code (and the removal
of block, convert, and bitblt), the interface to the Nova has been revised.
OSStaticDefs
The format of the OS statics region has been revised to reflect the changes in OS version 14
(see the Alto Operating System Reference Manual).
Pair Instructions
A number of the pair instructions described in the PrincOps have been implemented on the
Alto (notably the RXLP, RILP, and RIGP families).
Processes and Monitors
Due to severe space limitations, all of the process/monitor opcodes (enter, wait, reenter,
notify, broadcast, exit, and requeue) are implemented in Nova code, and therefore are
considerably slower (relative to other instructions) than they will be on the DO.
Real Data Type
The compiler now generates KFCBS to perform real arithmetic. The SD contains entries for
the following floating point operations: FADD, FSUB, FMUL, FDIV, FCOMP, FIX, FLOAT. Note,
however, that no implementation of these operations is provided or planned.