TweetFollow Us on Twitter

Technical Questions 2.1
Volume Number:2
Issue Number:1
Column Tag:Ask Prof. Mac

Readers Technical Questions

By Steve Brecher, Software Supply, MacTutor Contributing Editor

SCSI

Q. It's widely rumored that Apple is going to offer an upgrade that includes a "SCSI port." What's that?

A. At this writing Apple has made no announcements about such an upgrade, but assuming that the rumors are true let's talk about SCSI. This is just a background briefing -- if the Mac does get a SCSI connection, the only programmers who will need to be concerned with its details are those who write device driver and other low-level software to talk to the SCSI devices that attach to the Mac.

SCSI stands for Small Computer Systems Interface and is usually pronounced as "scuzzy." It is a standard 8-bit-wide data bus that is increasingly popular in the industry for connecting peripheral devices to each other and to host computers.

SCSI is an enhancement and standardization of SASI (Shugart Associates Systems Interface), a disk controller interface which was originated by Shugart, the disk drive manufacturer, in the late '70s and widely used. SASI was submitted to ANSI (American National Standards Institute) for official standardization. In the process its name was changed to drop the association with a particular vendor, and protocols were added to enable multiple hosts to coexist on the bus, to allow peripherals to release the bus during their execution of long processes and then reconnect with the host which issued the command, and to add commands oriented to devices other than disks (e.g., tape drives).

The basic purpose of SCSI, like that of other buses (S100, VME, Qbus, Multibus, etc.) is to connect devices whose manufacturers may never have heard of one another so that the devices can send data back and forth in a coordinated way. SCSI specifies both the electrical aspects of the connection and a communication protocol. Up to 8 devices (peripheral controllers and/or computers) can be connected to a SCSI bus.

The bus consists of 8 data lines ("wires"), one data parity line, and 9 control signal lines. (For reasons best known to the hardware folks, a 50-line cable is usually used for SCSI interconnections.) Data transfer is asynchronous -- that means that an explicit control signal handshake protocol is used to coordinate the transfer of each byte (as opposed to synchronous, which means that the communicating devices use a common clock signal to pace data transfers).

The maximum SCSI data transfer rate is more than 1MByte/sec -- thus fast peripherals will have to wait for the Mac rather than vice-versa. The Mac, lacking DMA (Direct Memory Access), cannot transfer more than about 0.5Mbyte/sec into and out of RAM, because each byte transferred (recall that SCSI is an 8-bit-wide data bus) requires at least one CPU instruction and two 68000 bus accessess -- data source and destination -- in addition to the access for the instruction fetch. That's not counting any additional instructions such as for loop control.

A typical SCSI bus system is shown in Figure 1. This is a single-host system -- only one computer -- which allows the computer to drive the peripherals connected to the bus. The illustration shows only two peripherals, but there could be up to 7 (the host is one of the up to 8 devices that can attach to the bus).

The host adapter in the computer interfaces between the SCSI bus and the CPU. CPUs like the 68000 which use memory-mapped I/O would communicate with the host adapter by writing to or reading from fixed addresses -- just as the Mac communicates with its serial and diskette hardware at fixed addresses.

The bus allows any one pair of devices connected to it to be in communication at a given time. One of the devices (usually the computer) is the Initiator and the other (usually a peripheral) is the Target. The Initiator obtains use of the bus, and selects a Target; the Target responds to indicate it is selected, and then the two devices have exclusive use of the bus until they release it The Initiator sends commands to the Target, and the Target executes them. The set of possible commands includes Read (send data to me), Write (take the following data), Format (a disk), Rewind (a tape drive), etc.

SCSI peripheral controllers are fairly intelligent. For example, to read from a disk device, the Initiator (computer) need specify only a logical block address (a relative data block offset) and the number of blocks. The target controller then translates this into a low-level disk location (cylinder, head, sector). Historically, driver software in the computer has been responsible for such translation to low-level terms, because that's all that "dumb" disk controllers were able to understand. In addition to making things simpler for the host computer device driver software, this kind of controller intelligence allows a higher degree of device-independence -- the same way that a high-level programming language allows CPU-independence.

Soft Coding

Q. "ELL" on Delphi notes that Apple often warns against "hard-coding" constants such as screen and memory sizes into programs, but some example programs from Apple seem to include such constants nonetheless.

A. Do as they say, not as they do. Any program that's going to be used more than once (or farther into the future than five minutes) should be defensively programmed to assume that anything about its environment that might change, will change. Even today a program might be run on machines varying in memory size from 128K to 2MB, and varying in screen size from the Mac's screen to the XL's.

Handling an arbitrary screen size can be difficult with respect to multiple windows which contain many elements and which have been designed to look pretty. But that's no excuse for avaoiding simple measures, such as calculating at run time instead of hard-coding the positions of a window which should be centered.

To get the lazy started, I've provided in Figure 2 a couple of routines to vertically and horizontally center a rectangle. The code is trivial, but it illustrates the use of a couple of macros discussed in the next question.

In those cases in which constants must be used, they should be defined mnemonically (e.g., using EQU in assembler or Const in Pascal). That makes them easy to identify and change.

Macros

Q. I haven't seen many macros used in the assembly programs I've looked at. Could you give some examples of macro usage?

A. I have a file of macro definitions that I incorporate into most programs using the MDS Asm Include facility. The file is shown in Figure 3.

The first set of macros compensates for an MDS bug which generates incorrect results for some comparison operators. For example, instead of

If A<B

which doesn't work properly, I use the macro .LT.:

If A .LT. B

(I stole the substance of these comparison macros from the TMON user area source code.)

The second and most elaborate set of macros is used to avoid errors in stack addressing for a routine's arguments, result, and temporaries. Figure 4 shows an example of how this set of macros is used.

The Assume macro is useful for explicitly stating assumptions made by the code (usually relating to the value of an offset which is defined in an Equ file, or to the adjacency of values accessed with autoincrement or autodecrement addressing). For example, the code in Figure 2 contains:

  Assume   top=0
 Add        (A0),D0

Coding top(A0) wastes a word if top=0; including the Assume macro in this case makes clear what is going on (as well as protecting the program from undetected error should the offset of "top" ever change).

The various Push and Pop macros reduce the amount of typing required to code pushes and pops of the A7 stack as well as making the program easier to read (at least to my eye).

The BitDefinitions and Bit macros make it easy to define mnemonic values for bit numbers and masks which are used with flag bytes and hardware registers. Coding

 BitDefinitions   InterruptFlags
 Bit                  OneSec
 Bit                  VertBlank
 Bit                  KbdRdy
 ;etc.

is the equivalent of coding

 OneSecBit      Equ 0
 OneSecMask      Equ 1
 VertBlankBit   Equ 1
 VertBlankMask   Equ 2
 KbdRdyBit      Equ 2
 KbdRdyMask      Equ 4
 ;etc.

spExtra

Q. Bill Bynum of Williamsburg, VA noticed that both Inside Macintosh and the June '85 issue of MacTutor (p. 45) indicate that the spExtra field of a grafPort record is a word, while in the MDS file QuickEqu.Txt, it's shown as a long (Pascal data type "fixed"). There is a corresponding discrepancy in the total size of a grafPort.

A. When in doubt, always go by the Equ files -- they're used by the folks at Apple who write systems software.

SpExtra is 4 bytes long, and a grafPort is 108 bytes long. Theoretically spExtra is of "fixed" data type, i.e., a 32-bit value with an implicit binary point in the middle. This implies that that spaces can be extended by a fractional number of pixels. However, in the current ROM QuickDraw effectively uses only the high-order word of spExtra. That's why the SpaceExtra trap works with an integer argument as it's documented in IM . QuickDraw moves a long argument from the stack to the field in the grafPort. Since the caller pushed only a word argument, the low-order word of the spExtra field in the grafPort will be whatever word happened to be above the caller's word argument on the stack. But since the low-order word doesn't really affect QuickDraw's calculations, all is well.

Figure 2 Source Code
 IncludeQuickEqu.D
 IncludeMacros ;see Figure 3

MBarHt  Equ 20 ;vertical size of menu bar
;
; procedure VertCenterRect(VAR myRect: rect)
;
; Adjust the top and bottom of the rect so that the rect is
; vertically centered in the desktop area for current
; screen size.
;
 StackFrame NotLinked
 Arg    myRect,Long
;
VertCenterRect:
 Move.L (A5),A0  ;addr of QuickDraw globals
 Move screenBits+bounds+bottom(A0),D0 
 ;vertical size of screen
 Move.L myRect(SP),A0;addr of caller's rect
 Move bottom(A0),D1;D1 = bottom of caller's rect
 Sub  D1,D0 ;D0 = old bottom margin
 Assume top=0  ;(macro)
 Add  (A0),D0  ;+ old top margin, D0 = total 
 ;vertical margin
 Lsr  #1,D0 ;D0 = 1/2 of the vert space 
 ;not used by rect
 Add  #MbarHt/2,D0 ;adjust for menubar, D0 = 
 ;new top of rect
 Sub  (A0),D1  ;D1 = vertical size of rect
 Move D0,(A0)  ;set new top
 Add  D1,D0 ;new bottom = top + size
 Move D0,bottom(A0);set new bottom
 Return ;(macro)
;
; procedure HorzCenterRect(VAR myRect: rect)
;
; Adjust the left and right of the rect so that the rect is
; horizontally centered for current screen size.
;
 StackFrame NotLinked
 Arg    myRect,Long
;
HorzCenterRect:
 Move.L (A5),A0  ;addr of QuickDraw globals
 Move screenBits+bounds+right(A0),D0 
 ;D0 = horz size of screen
 Move.L myRect(SP),A0;addr of caller's rect
 Move right(A0),D1 ;D1 = right of caller's rect
 Lea  left(A0),A0;point to left coordinate 
 ;of rect
 Sub  D1,D0 ;D0 = old right margin
 Add  (A0),D0  ;+ old left margin, D0 = 
 ;total horz margin
 Lsr  #1,D0 ;D0 = 1/2 of the horz space 
 ;not used by rect
 Sub  (A0),D1  ;D1 = horizontal size of rect
 Move D0,(A0)  ;set new left
 Add  D1,D0 ;new right = left + size
 Move D0,right-left(A0)   ;set new right
 Return ;(macro)
Figure 3
; Macros.Asm -- General purpose MDS macros

;
; Macros to work around MDS Asm comparison reversal bug
;
If 1<0  ;if bug is present
 Macro  .LT. = > |
 Macro  .LE. = >=  |
 Macro  .GT. = < |
 Macro  .GE. = <=  |
Else    ;if bug is not present
 Macro  .LT. = < |
 Macro  .LE. = <=  |
 Macro  .GT. = > |
 Macro  .GE. = >=  |
Endif

; Subroutine stack frame definition macros
;
; Usage:
;
;StackFrame Linked ;if A6 link to be used
;<or>
;StackFrame <anything else> ;if no A6 link to be used
;
;Arg    ArgN,ArgNLen ;last arg pushed by caller
;...
;Arg    Arg1,Arg1Len ;first arg pushed by caller
;Result ResultName,ResultLen
;Local  Local1,Local1Len
;...
;Local  LocalN,LocalNLen
;
;Routine:
;Link   A6,#-LocalsSize ;if StackFrame Linked
;...
;Return
;
; Notes:
;
; StackFrame is required.  Each of the other types of 
; macro invocations is optional, but if present their 
;relative ordering must be as shown.
;
; Arguments (Arg macro invocations) must appear in 
; the reverse of the order in which the caller pushes 
; the arguments.
;
; ResultLen is ignored, but provided for documentary 
; purposes.

Macro StackFrame Type =
 If'{Type}' = 'Linked'
 ..RtnAddr..Set  4
 ..ArgOffs..Set  8
 Else
 ..RtnAddr..Set  0
 ..ArgOffs..Set  4
 Endif
 ..ArgsSz.. Set  0
 LocalsSize Set  0
 |

Macro Arg Name,Len =
 {Name} Set ..ArgOffs..
 ..ArgOffs..Set  {Name}+{Len}+({Len}&1)
 ..ArgsSz.. Set  ..ArgOffs..-..RtnAddr..-4
 |

Macro ResultName,Len =
 {Name} Set ..ArgOffs..
 |

Macro Local Name,Len =
 {Name} Set 0-LocalsSize-{Len}-({Len}&1)
 LocalsSize Set  0-{Name}
 |

Macro Return =
 If..RtnAddr.. =4
 Unlk   A6
 Endif
 If..ArgsSz..  <>0
 Move.L (SP)+,A0 ;return addr
 If..ArgsSz..  .LE. 8
 AddQ   #..ArgsSz..,SP
 Else
 Lea    ..ArgsSz..(SP),SP
 Endif
 Jmp    (A0)
 Else
 Rts
 Endif
 |

;
; Define standard lengths
;
Byte  Equ 1      ;same affect as Word
 ; use for declarative purpose
Word  Equ 2
Long  Equ 4

;
; Macros to push and pop stack
;
Macro Pop.B Dest =
 If   '{Dest}'   <> ''
 Move.B (SP)+,{Dest}
 Else
 Tst.B  (SP)+  ;pop and set condition codes 
 ;per Boolean
 Endif
 |

Macro Pop Dest =
 If   '{Dest}'   <> ''
 Move.W (SP)+,{Dest}
 Else
 AddQ #2,SP
 Endif
 |

Macro Pop.L Dest =
 If   '{Dest}'   <> ''
 Move.L (SP)+,{Dest}
 Else
 AddQ #4,SP
 Endif
 |

Macro Push.BSrc =
 Move.B {Src},-(SP)
 |

Macro PushSrc =
 Move.W {Src},-(SP)
 |

Macro Push.LSrc =
 Move.L {Src},-(SP)
 |

Macro PushM Regs =
 MoveM.W{Regs},-(SP)
 |
 
Macro PushM.L  Regs =
 MoveM.L{Regs},-(SP)
 |

Macro PopMRegs =
 MoveM.W(SP)+,{Regs}
 |

Macro PopM.LRegs =
 MoveM.L(SP)+,{Regs}
 |

; Macro to make an assertion about an assumed condition
;
Macro Assume Cond =
 If   {Cond}
 Else
 Assumption error -- {Cond}
 Endif
 |
; Macros to decare bits and masks
; Usage
;BitDefinitions ID ;setup -- ID is documentary only 
;Bit  Name1 ;Defines Name1bit=0, Name1mask=1
;...
;Bit  NameN ;Defines NameNbit=N, NameNmask=1<<N
;
Macro BitDefinitions ID
 .BitNbr. Set  0
 |
Macro Bit Name =
 {Name}bitEqu  .BitNbr.
 {Name}mask Equ  1<<{Name}bit
 .BitNbr. Set  {Name}bit+1
 |
Figure 4
 IncludeSysEqu.D
 IncludeMacTraps.D
 IncludeMacros
;
; Function CanGetInfo(vRefNum, DirID: integer; FileNamePtr: 
 Ptr): boolean
; Calls _HGetFileInfo (HFS version) if DirID<>0, 
; otherwise _GetFileInfo.
; Returns true if the trap result is noErr.
;
ioHFQElSize Equ  $6C         ;size of HFS parameter block
ioDirID Equ $30          ;offset of DirID
; 
 StackFrame Linked
 Arg    FileNamePtr,long
 Arg    DirID,word
 Arg    vRefNum,word
 Result Flag,byte
 Local  ioPB,ioHFQElSize
;
CanGetInfo:
 Link A6,#-LocalsSize
 Lea  ioPB(A6),A0;addr of parameter block
 Move.L FileNamePtr(A6),ioFileName(A0)
 Move vRefNum(A6),ioVRefNum(A0)
 Clr.B  ioFVersNum(A0)  ;version number always 0
 Clr  ioFDirIndex(A0);not an indexed call
 Move DirID(A6),D0 ;HFS?
 Bne.S  @0;yes
 _GetFileInfo    ;no
 Bra.S  @1
@0 Move D0,ioDirID(A0)
 _HGetFileInfo
@1 Assume noErr=0
 Seq  Flag(A6)   ;set result
 Return
 
AAPL
$111.78
Apple Inc.
-0.87
MSFT
$47.66
Microsoft Corpora
+0.14
GOOG
$516.35
Google Inc.
+5.25

MacTech Search:
Community Search:

Software Updates via MacUpdate

NeoOffice 2014.6 - Mac-tailored, OpenOff...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
LibreOffice 4.3.5.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
CleanApp 5.0.0 Beta 5 - Application dein...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Monolingual 1.6.2 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
NetShade 6.1 - Browse privately using an...
NetShade is an Internet security tool that conceals your IP address on the web. NetShade routes your Web connection through either a public anonymous proxy server, or one of NetShade's own dedicated... Read more
calibre 2.13 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Mellel 3.3.7 - Powerful word processor w...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
ScreenFlow 5.0.1 - Create screen recordi...
Save 10% with the exclusive MacUpdate coupon code: AFMacUpdate10 Buy now! ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your... Read more
Simon 4.0 - Monitor changes and crashes...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
BBEdit 11.0.2 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more

Latest Forum Discussions

See All

Galaxy Trucker Pocket (Games)
Galaxy Trucker Pocket 1.0.8 Device: iOS iPhone Category: Games Price: $2.99, Version: 1.0.8 (iTunes) Description: Galaxy Truckers Wanted!================================================================= (5/5) "Galaxy Trucker isn’t... | Read more »
Make your own Tribez Figures (and More)...
Make your own Tribez Figures (and More) with Toyze Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
So Many Holiday iOS Sales Oh My Goodness...
The holiday season is in full-swing, which means a whole lot of iOS apps and games are going on sale. A bunch already have, in fact. Naturally this means we’re putting together a hand-picked list of the best discounts and sales we can find in order... | Read more »
It’s Bird vs. Bird in the New PvP Mode f...
It’s Bird vs. Bird in the New PvP Mode for Angry Birds Epic Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Telltale Games and Mojang Announce Minec...
Telltale Games and Mojang Announce Minecraft: Story Mode – A Telltale Games Series Posted by Jessica Fisher on December 19th, 2014 [ permalink ] | Read more »
WarChest and Splash Damage Annouce Their...
WarChest and Splash Damage Annouce Their New Game: Tempo Posted by Jessica Fisher on December 19th, 2014 [ permalink ] WarChest Ltd and Splash Damage Ltd are teaming up again to work | Read more »
BulkyPix Celebrates its 6th Anniversary...
BulkyPix Celebrates its 6th Anniversary with a Bunch of Free Games Posted by Jessica Fisher on December 19th, 2014 [ permalink ] BulkyPix has | Read more »
Indulge in Japanese cuisine in Cooking F...
Indulge in Japanese cuisine in Cooking Fever’s new sushi-themed update Posted by Simon Reed on December 19th, 2014 [ permalink ] Lithuanian developer Nordcurrent has yet again updated its restaurant simulat | Read more »
Badland Daydream Level Pack Arrives to C...
Badland Daydream Level Pack Arrives to Celebrate 20 Million Downloads Posted by Ellis Spice on December 19th, 2014 [ permalink ] | Read more »
Far Cry 4, Assassin’s Creed Unity, Desti...
Far Cry 4, Assassin’s Creed Unity, Destiny, and Beyond – AppSpy Takes a Look at AAA Companion Apps Posted by Rob Rich on December 19th, 2014 [ permalink ] These day | Read more »

Price Scanner via MacPrices.net

Holiday sale: 13-inch 128GB MacBook Air for $...
 Best Buy has the 2014 13-inch 1.4GHz 128GB MacBook Air on sale for $849.99, or $150 off MSRP, on their online store. Choose free home shipping or free local store pickup (if available). Price valid... Read more
13-inch 2.6GHz Retina MacBook Pro on sale for...
Best Buy has lowered their price on the 2014 13″ 2.6GHz/128GB Retina MacBook Pro to $1149.99 on their online store for a limited time. That’s $150 off MSRP and the lowest price available for this... Read more
Kodak Returns to CES With New Consumer Produ...
Former photography colossus Kodak is returning to CES for the first time in three years where the Kodak booth (#21818 South Hall 1) will showcase a wide range of innovative, imaging-related products... Read more
Invaluable Launches New Eponymously -Named A...
Invaluable, the world’s largest online live auction marketplace, hhas announced the official launch of the Invaluable app for iPad, now available for download in the iTunes App Store. Invaluable... Read more
IDC Reveals Worldwide Mobile Enterprise Appli...
International Data Corporation (IDC) last week hosted the IDC FutureScape: Worldwide Mobile Enterprise Applications and Solutions 2015 Predictions Web conference. The session provided organizations... Read more
Hello Vino Wine App Launches “Safe Ride Home”...
Hello Vino has announced addition of a new “Get a Safe Ride Home” feature in its Food & Drink app with a direct connection to Uber, the technology platform that connects users with rides. The... Read more
DEVON-technologies Releases DEVONthink To Go...
Coeur d’Alene, Idaho based DEVON-technologies, LLC has updated DEVONthink To Go, its mobile companion to DEVONthink, to version 1.5. The update includes an iOS 8 extension, compatibility with the... Read more
The Apple Store offering free next-day shippi...
The Apple Store is now offering free next-day shipping on all in stock items if ordered before 12/23/14 at 10:00am PT. Local store pickup is also available within an hour of ordering for any in stock... Read more
It’s 1992 Again At Sony Pictures, Except For...
Techcrunch’s John Biggs interviewed a Sony Pictures Entertainment (SPE) employee, who quite understandably wished to remain anonymous, regarding post-hack conditions in SPE’s L.A office, explaining “... Read more
OtterBox Defender Series Case For iPad mini 3...
With their innovative Touch ID technology and ultrathin profile, the latest tranche of Apple iPads are more desirable than ever, and OtterBox has just announced the Defender Series custom-engineered... Read more

Jobs Board

*Apple* Store Leader Program (US) - Apple, I...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on experience, Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.