Imaging a PowerPC with an Intel-Based Mac
Creating the Image
Setup the your PowerPC as you like it with all the software, updates, preferences, and user accounts that you like. Once you have your PowerPC all ready to image, restart the computer. Before the computer chimes with the Mac start up sound, hold down the 't' key to boot the computer into Target Disk Mode. This mode will allow you to access file and architecture specific things while the operating system isn't running. Connect the PowerPC with a FireWire cable to the Intel-Based Mac. Once connected, usually time machine pops up with a dialog box asking if you want to use that drive for backups. Always close those windows when they pop up, they are not the type of backup that is needed in this case.
On the Intel-Based Mac, open the application called "Disk Utility". You can find it two ways 1) by spot light search, pressing the command+space will open it by default, and 2) by navigating to Applications->Utilities->Disk Utility. Disk Utility displays all mounted disk to the running computer. Therefore, you should see the hard disk from the computer you are running, the PowerPC that you are trying to image, and any other disk that might be attached like flash drivers, or external drives.
Click on the FireWire Target Media drive. Once that drive is highlighted, click the icon that says "New Image" at the top. A dialog box will appear and ask you were to save it, the file name, compression type, and encryption. In this situation, set compressing type to read-only and encryption to none, then save.
Once the image is created, we can start imaging other computers with that image. Unmount the computer that is in Target Disk Mode when you are finished.
Resorting the Image
To resorting an already made image (dmg file) to another PowerPC from our Intel-Based Mac. Connect the computer through Target Disk Mode, as before by holding down 't', and connect it to your Intel-Based Mac, then open Disk Utility on the Intel-Based Mac. First we have to erase what is on the computer to start clean. To do this, click on the FireWire Target Media and navigate to the "Partition" tab. Under Volume Scheme: 1 Partition and Name: Macintosh HD, then under options make sure that the partition scheme is Apple Partition Map, since only this will allow the computer to boot correctly (It will not work any other way). Then click "Apply".
Once partitioned, navigate to the Restore tab. Drag and drop the dmg image you made earlier, or click Image to locate it, to the source box. The destination will be the newly partitioned drive, this also can be drag and dropped. Check the box "Erase Destination" and click Restore. The process ranges on time depending on how large the image is.
After the restore is complete, unmount the drive, then restart the computer. The computer will be exactly like the image that was made.
Cyclic Redundancy Check (CRC) Calculation
Cyclic redundancy check (CRC) codes are an error-detection technique that is widely used in today's computer networks. Consider a d-bit piece of data, denoted as D, that will be sent from one node to the other. Both nodes must have an agreed upon an r+1 bit pattern, known as a generator, which will be denoted as G. However, there is a requirement for G that the leftmost bit has to be a one. We will also denote D' being the original D times 2 to the power r (D*2^r). The goal of CRC is to tack on r additional bits to D, these r-bits will be the remaining bits when G is divided into D', denoted by R (the remainder). When the R is added to D', that will be the bit pattern that is exactly divisible with G using modulo-2 arithmetic.
To show this process more clearly, we will walk through an example.
Suppose G = 10011 and D = 1010101010
- Since G is an agreed upon from both nodes, it will be given. Since G is 5 bits long, then r is |G|-1=4, r=4.
- D will be now shifted left by 4-bits and zero will be inserted into those places, and the new pattern will be denotes by D' = 10101010100000
- Now we will divide G into D' using an "exclusive or" operation. This is shown in Figure 1.
- The remainder from the division, R, will be the bit pattern need to add to D' such that the resulting bit pattern will be exactly divisible with G.
Figure 1: Initial Division
To divide one binary number into another (as shown in Figure 1 and 2). The exclusive or operation is used. If you don't know how to compute this type of operation consider the example:
Suppose you would like to take the exclusive or of these two binary numbers: 10101 10011
First compare each number in the same positions in the two strings starting from the right to the left.
10101
10011
The first compare is between 1 and 1: an exclusive or between the two will result in 0. This is because they are the same number and the operation between the two will result in false. In exclusive or you can only have one or the other, not both.
The second comparison is between 0 and 1: an exclusive or between the two will result in 1. This is because they are both different numbers, therefore will result in true.
The fourth comparison is between 0 and 0: an exclusive or between the two will result in 0 for the same reason that the first comparison was a 0.
You should continue these comparisons for all the bits in the string, and result with 00110 or just 110.
If you are not quite sure whether you computed the correct remainder R. This is a simple way to check your work.
- From the example about the resulting bit pattern when R is added to D' is D'' = 10101010100100
- Then divide G into D''.
- If you computed the correct R then the resulting remainder will be 0. This is shown in Figure 2.
Figure 2: Division Check
