TweetFollow Us on Twitter

I/O Routines
Volume Number:1
Issue Number:6
Column Tag:FILE SAVING, OPENING

I/O Completion Routines

By Steve Brecher

I/O Completion Routines

Application I/O requests are ultimately performed by low-level operating system trap routines: _Read, _Write, _Control, _Status, _Open, _Close. The program passes the address of an I/O parameter block to the trap routine. The parameter block specifies the device driver (e.g., Sony floppy driver, serial driver) and the particulars of the request such as (for _Read or _Write) the number of bytes and the address of the program’s buffer.

The operating system passes the parameter block on to the device driver; or, if the driver is currently busy, the parameter block is put onto the driver’s queue of pending work.

Mac I/O operations can be either synchronous or asynchronous. A synchronous operation will be completed before control is returned to the invoking program. An asynchronous operation will be initiated -- started upon by the driver, or put into its queue-- and then control will be returned to the program regardless of whether the operation is yet complete.

Asynchronous I/O is the heart of multi-tasking; indeed, the only rationale for multi-tasking is the overlapping of I/O and computation. Programs can imp- lement a primitive form of multi-tasking within themselves by using a multi-buffer I/O scheme. For instance, a program can request that a buffer full of data be written, and then -- before that write request is completed -- immediately start filling another buffer with data.

Completion routines can be a convenient mechanism for a program to manage multi-buffered I/O. The com- pletion routine is called asynchronously with respect to the rest of the program; it can mark the buffer associated with the just-completed I/O request as free (written) or ready for processing (read). If another buffer is ready to be read into or written from, the completion routine can initiate the next I/O request.

Completion routines can also be used to hide low-level I/O details from the mainline logic of a program. I attempted to use the Mac’s completion routine facility for that purpose in a program doing input from the modem port at high baud rates. The program started off by issuing an asynchronous read request for one byte, and specifying a completion routine. The completion routine looked like this:

-- Put the received byte into program’s input buffer;

-- Issue a _Status call to find out whether the serial input driver has more data in its own buffer;

-- If the driver has more data available, issue a _Read to transfer it to the program’s input buffer;

-- Re-issue the one-byte asynchronous _Read.

The idea was for the completion routine to handle the low-level details of managing input into the program’s buffer. The mainline of the program would then be freed from concern with those details, and could just take data from the buffer as it became available.

But this scheme didn’t work. Following is an explanation of why it didn’t, which relates to a severe limitation in the Mac’s implementation of completion routines.

Asynchronous I/O is requested by setting bit 10 ($0400) in the trap value. There are two ways for a program to detect the completion of asynchronous I/O. When an I/O operation is initiated, the ioResult field of the parameter block is set to 1; when the operation completes, ioResult is set to 0 (if no error) or to a negative error code. So the program may test ioResult to determine whether the operation is complete. Or, the program may specify the address of an I/O completion routine in the ioCompletion field of the parameter block. If ioCompletion is not nil (zero), then when the operation is complete the OS will JSR to the address contained in ioCompletion.

If bit 10 of the trap word is clear -- a synchronous I/O -- the Device Manager will clear ioCompletion before initiating the I/O; and the OS will test ioResult in a tight loop, waiting for it to become zero or negative before returning from the trap. Hence, you don’t need to clear ioCompletion prior to a synchronous I/O; but for asynchronous I/O you must either either clear ioCompletion or set it to the address of a completion routine.

When a completion routine is entered, register D0.W contains the ioResult value (and the condition codes reflect a TST of its value); A0 contains the parameter block address; 4(SP) -- just above the return address -- contains a pointer to the driver’s Device Control Entry (DCE); and registers D0-D3/A0-A3 may be altered by the completion routine. The I/O queue element (a.k.a. the parameter block) will have been unlinked from the driver’s queue. And -- here’s the catch -- interrupts may or may not be disabled.

The OS initiates an I/O operation by calling the appropriate entry point of the device driver. Let’s consider, for example, a read request to the serial input driver. The OS calls the serial input driver’s “Prime” (read) routine. There are two possibilities: either the request will be completed immediately by the Prime routine, or it will be completed later as the result of an interrupt.

If the number of characters in the driver’s buffer is equal to or greater than the number specified in the I/O request, the Prime routine will fulfill the request immediately by transferring characters to the caller’s ioBuffer, and then jump to the OS ioDone routine, which will call the completion routine. Interrupts are enabled at entry to the completion routine, and it can therefore do anything that any other part of an application can do. In this case, when the completion routine is entered the original _Read trap has not yet returned and no other application code has been executed. Therefore the effect is similar to that of the application executing the following:

     _Read                          ;synchronous
     JSR    CompletionRoutine

If the completion routine initiates another I/O request to the same driver, specifying the same ioCompletion (i.e., itself), that request may also complete immediately in which case the completion routine will be re-entered.

If there are insufficient characters in the driver’s buffer to satisfy a read request, the Prime routine will return to the OS, which will return from the trap. Later, the driver’s received-character interrupt service will determine that the request is complete, and jump to the OS ioDone routine, which will call the completion routine. Now, however, the completion routine is entered with interrupts disabled; in effect the completion routine is part of the interrupt service.

The completion routine can do no significant processing, because that would hold off interrupts for too long a time -- for example, subsequent incoming serial data might be lost. This virtually excludes the possiblity of intitiating a new I/O operation from within the completion routine. Also, the completion routine (at interrupt level) cannot do anything that would alter the heap configuration -- the interrupt might have occurred while a handle was being dereferenced.

Since there is no point to doing asynchronous I/O unless it is assumed that the request will not complete immediately, the programmer must assume that a serial I/O completion routine will be executed at interrupt level. This makes the completion routine facility rather useless for serial I/O. The routine could set a flag indicating the completion -- but such a flag is already available in ioResult.

Wish list item... To make completion routines useful, the Mac OS would have to be enhanced to implement what the DEC PDP-11 operating systems refer to as “fork processes.” Fork processes are serialized and executed synchronously after all (possibly nested) interrupts have been dismissed, but before control is returned to the point at which the first interrupt occurred. This enables completion routines to be inter- ruptable, and gives them time to do useful work.

MS BASIC programmers: Stuff it!

The QuickDraw StuffHex routine is a fast way to convert lengthy hex machine language data to binary code in MS BASIC programs. The trick is to prefix each string of hex digits with a character whose ASCII value is equal to the number of hex digits: this character is thus a length byte, making the string a Pascal string which can be passed to StuffHex. Note that the address of the string data is not given by the address of the BASIC string variable. The string variable is actually a data structure containing information about the string, including its address. The address can change each time the string is assigned or read into. We get the address of the string data by PEEKing into the string variable. Example:

‘
‘ Machine language interface to StuffHex:
‘
DIM SH%(2)
SH%(0)=&H245F : SH%(1)=&HA866
SH%(2)=&H4ED2
‘ Array that gets the binary machine
‘ language:
DIM CODE%(SomeAdequateSize)
‘ Declare all scalars before getting array
‘ addresses:
CODEPTR!=0! : HEXLINE$=”” :  STRINGPTR!=0!
P!=0!
‘ Get addresses of arrays and the string
‘ variable:
STUFFHEX!=VARPTR(SH%(0))
CODEPTR!=VARPTR(CODE%(0))
P!=VARPTR(HEXLINE$)
‘ Read lines of hex data, convert to binary:
READ HEXLINE$
WHILE HEXLINE$<>””
  ‘ Get address of first byte of string data
  ‘ (the length byte):
  STRINGPTR!=(PEEK(P!+2!)*65536!)
               +(PEEK(P!+3!)*256!)+PEEK(P!+4!)
  ‘ Convert the line’s hex data to binary:
  CALL STUFFHEX!(CODEPTR!,STRINGPTR!)
  ‘ Adjust pointer into CODE! array for next
  ‘ data (if any):
  CODEPTR!=CODEPTR!+(ASC(HEXLINE$)/2)
  READ HEXLINE$
WEND
‘
‘ The ASCII value of the first character
‘ of each string must be equal to the
‘ number of characters in the rest of the
‘ string.  Thus the number of hex digits
‘ in each string must be at least 32
‘ (ASCII space) and no greater than
‘ 126 (ASCII ~) so that the length is a
‘ displayable ASCII character.  But the
‘ length must be other than 34 -- 34 is
‘ the ASCII code for quotation mark!
‘ Note the following DATA line has 36 hex
‘ digits, and that the first character of the
‘ string is “$”, i.e., CHR$(36).
‘
DATA”$0123456789ABCDEF0123456789ABCDEF0123"
‘
‘ More lines of hex data would go here
‘ Empty string marks end of data:
‘
DATA “”

Reports from Miss Elaine E.

Due to a QuickDraw bug, DrawText in srcCopy mode will erase four or five character positions after the position of the last character in the string. It’s OK to use srcXor if no character position will be overwritten; but if you need srcCopy, a workaround is to set the right side of the port’s clipRegion to the right edge of the last character before calling DrawText. If the font is not monospace, this implies calling TextWidth first to find out the screen width of the string to be drawn.

The QuickDraw ScrollRect routine will be slowed by a factor or 3 or 4 if you include the borders of the GrafPort in the rect that is scrolled. Make sure the rect passed to ScollRect is inset at least a couple of pixels from the port’s border(s).

Thanks to Steve Hanna for this tip on the alternate screen buffer... To have QuickDraw draw in the alternate screen buffer, change the BaseAddr field of the GrafPort to $72700. When you use QD calls with that GrafPort as the current port, the drawing will be done in the alternate screen buffer. To flip the display between the main and alternate screen buffers, complement bit 6 of VIA buffer A, which is mapped to address $EFFFFE. (Steve Hanna also notes that the low-order 3 bits of buffer A control sound volume [0..7]. Bit 7 is tied to the SCC -- see below). The following instruction will flip the display to the other screen buffer:

Bchg.B  #6,$EFFFFE

Thanks to Dennis Brothers for pointing out that the MacsBug HD (heap display) command is a way find your program in memory -- the first CODE resource in the heap is most likely the first (or only) segment of your program.

The reason the ROM serial driver doesn’t support input flow control is that a D0 was coded where a D3 should have been; a two-bit error. And the reason mouse interrupts will be lost if you close the ROM serial driver (without immediately opening the RAM serial driver) is that the ROM serial driver neglects to finish its cleanup by setting the master interrupt enable bit in the SCC chip -- a one-word omission from a table. (Mouse movement signals come in through the DCD pins of the SCC and generate SCC external status interrupts.)

If you want to program the SCC yourself, ask your local Zilog office for a copy of the “Z8030/Z8530 SCC Serial Communications Controller Technical Manual.” The memory-mapped addresses of the the SCC are in the MDS equate file SysEquates.Txt. The SCC WAIT/REQUEST (asserted low) pin state is brought over to bit 7 of VIA (6522 chip) buffer A, which is mapped to memory address $EFFFFE. The serial driver’s Open routine configures the SCC to assert WAIT/REQUEST when an input character is available in the SCC buffer. This enables the Sony floppy driver to feed incoming data to the serial driver’s input buffer while the Sony driver has disabled interrupts. When the byte at $EFFFFE is positive (bit 7 clear), the Sony driver fetches the character waiting in the SCC and calls code in the serial driver interrupt service routine.

Inside Macintosh says of the serial status ctsHold flag (Serial Driver, p. 13), “If output has been suspended because the hardware handshake has been negated, ctsHold will be nonzero.” The conditional clause could be misleading: provided that the output driver has been opened, ctsHold always reflects the state of the CTS (asserted low) pin of the SCC (pin 7 of the DB-9 connector) regardless of the status (or existence) of any output request. If CTS is asserted, ctsHold is zero.

Need a quick (and dirty!) test of whether any OS events are pending? Address $014C contains the OS event queue header (a pointer); if it’s nil (zero), the queue is empty.

Think you have a good (homemade) backup of Macintosh Pascal? Make sure you can click the mouse 101 times in the source edit window. On the 101st click, Mac Pascal goes to the disk to check that the master is there; if it doesn’t like what it finds, it abrubtly quits to the Finder (trashing any of your unsaved work). In my book, this qualifies as a “worm” and, since it’s not documented, is tasteless at best and unethical at worst. If a publisher wants to frustrate users who attempt to backup a product or use it on a hard disk without inserting the master, the program should either quit at the outset or put up a dialog box demanding the master. The more experienced the programmer, the more violent his aversion to being forced to use distribution media in production work.

Consulair Mac C users who want all string constants to be compiled in Pascal format (length-byte prefix) can include the following at near the top of the source file:

 #asm
         STRING_FORMAT 3
 #endasm 

If there’s no semantic difference between pre-incrementing and post-incrementing a variable in your Mac C program (i.e., you can choose either ++i or i++), use pre-increment -- it generates more efficient code. Same applies to decrements. If you use post- inc(dec)rement, the generated code will inc(dec)rement the variable in memory, then offset that operation by dec(inc)rementing it in a register in preparation for its previous value being used in an expression -- even if it’s not so used.

A Mac C update is expected to be available this spring with floating point, register variables, structure assignments, and slicker code generation. Look for the Greenhills C compiler under the Apple name on the Mac later this year -- it’s being ported from the Lisa along with the Workshop. I haven’t used it, but it’s reported to generate slick code.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Trend Micro 11.0.2062 - An essential sec...
Trend Micro Antivirus provides essential security for macOS with real-time malware detection and mitigation in an affordable solution with a simple, intuitive interface. However, be aware that more... Read more
Backblaze 7.0.2.490 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $6 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
Pro Video Formats 2.2.1 - Updates for pr...
Pro Video Formats includes support for the following professional video codecs: Apple Intermediate Codec Apple ProRes AVC-Intra 50 / 100 / 200 / 4:4:4 / LT AVC-LongG XAVC XF-AVC DVCPRO HD HDV XDCAM... Read more
Boom 3D 1.3.11 - $19.99
Boom 3D is a revolutionary app with 3D Surround Sound and phenomenally rich and intense audio that is realistic and works on any headphones. Features 3D surround sound Built-in audio player... Read more
Final Cut Pro 10.5.2 - Professional vide...
Redesigned from the ground up, Final Cut Pro combines revolutionary video editing with a powerful media organization and incredible performance to let you create at the speed of thought.... Read more
Chromium 89.0.4389.72 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 89.0.4389.72: Complete list of changes can... Read more
iMovie 10.2.3 - Edit personal videos and...
With a streamlined design and intuitive editing features, iMovie lets you create Hollywood-style trailers and beautiful movies like never before. Browse your video library, share favorite moments,... Read more
OmniPlan 4.2.5 - Professional-grade proj...
With OmniPlan, you can create logical, manageable project plans with Gantt charts, schedules, summaries, milestones, and critical paths. Break down the tasks needed to make your project a success,... Read more
Compressor 4.5.2 - Adds power and flexib...
Compressor adds power and flexibility to Final Cut Pro X export. Customize output settings, work faster with distributed encoding, and tap into a comprehensive set of delivery features. Features:... Read more
Motion 5.5.1 - Create and customize Fina...
Motion is designed for video editors, Motion 5 lets you customize Final Cut Pro titles, transitions, and effects. Or create your own dazzling animations in 2D or 3D space, with real-time feedback as... Read more

Latest Forum Discussions

See All

Frogger in Toy Town's latest update...
Konami Digital Entertainment has announced today that their Apple Arcade title Frogger in Toy Town has been updated to introduce a new ranked Endurance Mode. This new game variant's arrival is also accompanied by a few other changes. [Read more] | Read more »
Mitoza is surreal adventure toy you can...
The folks behind the Rusty Lake games have just put a new title onto the App Store. Second Maze, Rusty Lake's collaborative publishing brand, has just brought this 10 year old adventure game from Gal Mamalya to mobile. The best part about all of... | Read more »
Pocket Gamer Awards 2021: You have five...
Three weeks ago our sister site, Pocket Gamer, entered the voting stage for the upcoming Pocket Gamer Awards 2021 and is now in the final stretch. You only have a few hours left to vote for the games you’ve enjoyed on mobile in the past year, as... | Read more »
Patty Stack is a casual arcade game, ava...
Patty Stack is a casual arcade title that's available now for iOS and Android. It's the debut game from developer Feeka Games tasks players with making an increasingly giant burger tower. Think of it as Tower Bloxx but more edible. [Read more] | Read more »
Distract Yourself With These Great Mobil...
There’s a lot going on right now, and I don’t really feel like trying to write some kind of pithy intro for it. All I’ll say is lots of people have been coming together and helping each other in small ways, and I’m choosing to focus on that as I... | Read more »
Genshin Impact Guide - Gacha Strategy: W...
This is part 2 of our Genshin Impact gacha strategy guides. See part 1 here. You can check out more guides for Genshin Impact here. | Read more »
Slashy Camp is a new endless runner insp...
Blue Wizard Digital has released Slashy Camp onto iOS and Android after it spent a short amount of time in early access. [Read more] | Read more »
Kinder World is a relaxing game about lo...
Lumi Interactive is releasing a game called Kinder World later this year on iOS and Android, which is all about looking after houseplants. [Read more] | Read more »
Steam Link Spotlight - Fights in Tight S...
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was on Hades. Read about how it plays using Steam Link over here. | Read more »
Lyxo, the light-based puzzler for mobile...
Vienna-based independent game studio Emoak has just released its unique light-based puzzler for iOS and Android. Founded in 2014 by Tobias Sturn, the company is also the creative force behind the infinite climbing game Paper Climb, as well as the... | Read more »

Price Scanner via MacPrices.net

Weekend Sale: $100 off Apple iPad Magic Keybo...
Amazon has Apple iPad Magic Keyboards on sale for $100 off MSRP for a limited time. Amazon’s prices are the lowest available for iPad Magic Keyboard from any Apple reseller this weekend: – Magic... Read more
Gazelle now offering a full line of refurbish...
Gazelle is now offering a full range of discounted, refurbished, unlocked Apple iPhone 12 models starting at $649. iPhones are offered in Fair, Good, and Excellent conditions, and multiple colors are... Read more
These are the latest discounted iPhones Apple...
Apple has a range of Certified Refurbished iPhones available right now starting at only $339. Apple includes a standard one-year warranty, new outer shell, and shipping is free. According to Apple, “... Read more
Save up to $64 on new M1 MacBook Airs at Expe...
Apple reseller Expercom has 2020 13″ M1 MacBook Airs on sale for $51-$64 off Apple’s MSRP with prices starting at $947.96. In addition to their MacBook Air sale prices, take $50 off AppleCare+ when... Read more
Discounts available on 16″ MacBook Pros with...
Upgrade a 16″ 6-Core or 8-Core MacBook Pro from 16GB of standard RAM to 32GB at Adorama, and save $100-$210 over Apple’s price for this custom option: – 16″ 6-Core MacBook Pro/32GB RAM: $2699, save $... Read more
10.9″ iPad Airs on sale for $50-$70 off Apple...
Amazon has new 2020 10.9″ Apple WiFi iPad Airs in stock and on sale today for up to $70 off MSRP with prices starting at $549. Note that Amazon’s sale price might be restricted to certain colors (see... Read more
Apple restocks 2020 27″ 5K iMacs for up to $3...
After an initial offering in January, Apple has restocked a full line of Certified Refurbished 2020 27″ 5K iMacs starting at $1529 and up to $350 off original MSRP. Apple’s one-year warranty is... Read more
Sale! 16″ 8-Core MacBook Pro for $2449, $350...
Apple reseller Adorama has the 16″ 2.3GHz 8-Core Space Gray MacBook Pro in stock and on sale today for $2449 including free shipping. Their price is $350 off Apple’s MSRP for this model, and it’s the... Read more
Roundup of 13″ Multi-Core Intel MacBook Pro s...
Apple resellers are offering significant sales & deals this week on 2020 13″ MacBook Pros with 10th generation Intel CPUs. Take up to $250 off Apple’s MSRP, get free fast shipping, and/or pay no... Read more
64GB iPhone 8 Plus available for $379 at Appl...
Apple has the 64GB iPhone 8 Plus in Space Gray & Gold colors available for $379 today, Certified Refurbished. Each phone is unlocked and comes with Apple’s standard 1-year warranty and free... Read more

Jobs Board

Geek Squad Advanced Repair *Apple* Professi...
**795178BR** **Job Title:** Geek Squad Advanced Repair Apple Professional **Job Category:** Store Associates **Store Number or Department:** 001406-Allen Park-Store Read more
Geek Squad *Apple* Consultation Professiona...
**796549BR** **Job Title:** Geek Squad Apple Consultation Professional **Job Category:** Store Associates **Store Number or Department:** 001800-Hot Springs-Store Read more
*Apple* Mobility Specialist - Best Buy (Unit...
**796014BR** **Job Title:** Apple Mobility Specialist **Job Category:** Store Associates **Store Number or Department:** 001776-Woodmore Towne Centre-Store **Job Read more
Systems Architect, *Apple* Production Engin...
…package beginning on your first day? If so, we hope you'll keep reading! The Apple Sales Engineering and account team is looking for a stellar presales engineer with Read more
Systems Engineer, Webscale, *Apple* Retail,...
…beginning on your first day? If so, we hope you'll keep reading! The Apple Sales Engineering team is looking for a pre-sales engineer with Enterprise engineering Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.