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
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Black Hole Joyrider (Games)
Black Hole Joyrider 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Guide your spacecraft through the gravity well of a powerful black hole using only your retro-thrusters and dwindling fuel... | Read more »
My Koi (Games)
My Koi 1.0 Device: iOS iPhone Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: My Koi is a beautiful and relaxing fish pond app. Customise and name each fish. Feed them daily. Watch them grow. Collect new fish.... | Read more »
Never Gone (Games)
Never Gone 1.0.2 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.2 (iTunes) Description: ###IMPPORTANT### Never Gone's HD art resources require devices with more than 1GB RAM, so please note that iPhone 4/4s, iPad 2/... | Read more »
INKS. (Games)
INKS. 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: From the makers of BAFTA-winning Lumino City comes INKS. INKS updates pinball for a new generation. It combines the joy of pinball with... | Read more »
How to maximise your profits in Bakery B...
Running a bakery can be an expensive venture. You’ll need to continuously upgrade your oven, your kitchen supplies, and even your ingredients to keep customers happy. Most of these renovations in Bakery Blitz cost a pretty penny, but we have a few... | Read more »
How to manage your time in Bakery Blitz
It can be tricky, especially when you risk burning your kitchen to the ground if you forget a cake in the oven, so make sure to use these time management tricks to keep your bakery running smoothly. Don’t collect the money right away [Read more] | Read more »
Model 15 (Music)
Model 15 1.0 Device: iOS iPhone Category: Music Price: $29.99, Version: 1.0 (iTunes) Description: The Moog Model 15 App is the first Moog modular synthesizer and synthesis educational tool created exclusively for iPad, iPhone and... | Read more »
How to deal with wind in Angry Birds Act...
Angry Birds Action! is a physics-based puzzler in which you're tasked with dragging and launching birds around an obstacle-littered field to achieve a set objective. It's simple enough at first, but when wind gets introduced things can get pretty... | Read more »
How to get three stars in every level of...
Angry Birds Action! is, essentially, a pinball-style take on the pull-and-fling action of the original games. When you first boot it up, you'll likely be wondering exactly what it is you have to do to get a good score. Well, never fear as 148Apps... | Read more »
The beginner's guide to Warbits
Warbits is a turn-based strategy that's clearly inspired by Nintendo's Advance Wars series. Since turn-based strategy games can be kind of tricky to dive into, see below for a few tips to help you in the beginning. Positioning is crucial [Read... | Read more »

Price Scanner via MacPrices.net

12-inch 128GB iPad Pros on sale for up to $10...
B&H Photo has 12″ 128GB WiFi iPad Pros on sale for up to $100 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 128GB WiFi iPad Pro: $899 $50 off MSRP... Read more
Global Tablet Sales Slump Continues, iPad’s F...
Another miserable showing for the global slate tablet category in calendar Q1/16, with global tablet shipments falling another 1ten percent to 46.5 million units during the according to Strategy... Read more
Revel Systems to Showcase iPad POS Platform w...
Revel Systems, specialists in iPad Point of Sale management solution for brick-and-mortar retail, food businesses and more, today announced that it will showcase its innovative iPad Point of Sale... Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Apple refurbished 2015 iMacs available for up...
Apple now has a full line of Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are... Read more
Indian Smartphone Market Grows Annually by 12...
India’s smartphone market grew by 12 percent year-over-year, with 24.4 million units shipping in Q1 2016. The top five vendors stayed the same, with Samsung in the lead, followed by Micromax, Intex... Read more
Get Notifications When Your Friend’s Phone Ba...
Calgary, Canada based Stonelight Pictures has announced the release of Battery Share 1.0.1, its new utility for iOS 9 supported devices. The company notes that people are spending more time on their... Read more
11-inch 1.6GHz/128GB MacBook Air on sale for...
Amazon has the current-generation 11″ 1.6GHz/128GB MacBook Air (sku MJVM2LL/A) on sale for $749.99 for a limited time. Their price is $150 off MSRP, and it’s the lowest price available for this model... Read more
Price drops on clearance 2015 13-inch MacBook...
B&H Photo has dropped prices on clearance 2015 13″ MacBook Airs by up to $250. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799, $200... Read more
Mac minis on sale for up to $100 off MSRP
B&H Photo has Mac minis on sale for up to $100 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP - 2.8GHz Mac mini... Read more

Jobs Board

*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
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.