Due to the high number of new COVID-19 infections in my home country last summer, I had to "improvise" yet another summer holiday. As a result, I finally found the time to tinker with my old computers again after a very long time of inactivity.
As I have explained in two blog posts that I wrote over ten years ago, the first computer (a Commodore 128 bought by my parents in 1985) and second computer (Commodore Amiga 500 bought by my parents in 1992) that I ever used, are still in my possession.
In the last few years, I have used the Commodore 128 a couple of times, but I have not touched the Commodore Amiga 500 since I wrote my blog post about it ten years ago.
It turns out that the Commodore Amiga 500 still works, but I ran into a number of problems:
- A black and white display. I used to have a monitor, but it broke down in 1997. Since then, I have been using Genlock device to attach the Amiga to a TV screen. Unfortunately, in 2021 the Genlock device no longer seems to work.
The only display option I had left is to attach the Amiga to a TV with an RCA to SCART cable by using the monochrome video output. The downside is that it is only capable of displaying a black and white screen. - No secondary disk drive. I used to have two 3.5-inch double density disk drives: an internal disk drive (inside the case) and an external disk drive that you can attach to the disk drive port.
The external disk drive still seems to respond when I insert a floppy disk (the led blinks), but it no longer seems to be capable of reading any disks. - Bad hard drive and expansion slot problems. The expansion board (that contains the hard drive) seems to give me all kinds of problems.
Sometimes the Amiga completely fails to detect it. In other occasions, I ran into crashes causing the filesystem to return me write errors. Attempting to repair them typically results in new errors.
After thoroughly examining the disk with DiskSalv, I learned that the drive has physical damage and needs to be replaced.
I also ran into an interesting problem from a user point of view -- exchanging data to and from my Amiga (such as software downloaded from the Internet and programs that I used to write) is quite a challenge. In late 1996, when my parents switched to the PC, I used floppy disks to exchange data.
In 2021, floppy drives have completely disappeared from all modern computers. In the rare occasion that I still need to read a floppy disk, I have an external USB floppy drive at my disposal, but it is only capable of reading high density 3.5-inch floppy disks. A Commodore Amiga's standard floppy drive (with the exception of the Amiga 4000) is only capable of reading double density disks.
Fortunately, I have discovered that there are still many things possible with old machines. I brought both my Commodore 128 and Commodore 500 to the Home Computer Museum in Helmond for repairs. Furthermore, I have ordered all kinds of replacement peripherals.
Getting it all to work, turned out to be quite a challenge. Eventually, I have managed to overcome all my problems and the machine works like a charm again.
In this blog post, I will describe what problems I faced and how I solved them.
Some interesting properties of the Amiga
I often receive many questions from all kinds of people who want to know why it is so interesting to use such an old machine. Aside from nostalgic reasons, I think the machine is an interesting piece of computer history. At the time the first model was launched: the Amiga 1000 in 1985, the machine was far ahead of its time and provided unique multimedia capabilities.
Back in the late 80s, system resources were very limited (such as CPU, RAM and storage) compared to modern machines, but there were all kinds of interesting facilities to overcome their design limitations.
For example, the original Amiga 500 model only had 1 MiB of RAM (512 KiB chip RAM and 512 KiB fast RAM) and 32 configurable color registers. Colors can be picked out of a range of 4096 possible colors.
Despite only having the ability to configure a maximum 32 distinct colors, it could still display photo-realistic images:
As can be seen, the screen shot above clearly has more than 32 distinct colors. This is made possible by using a special screen mode called Hold-and-Modify (HAM).
In HAM mode, a pixel's color can be picked from a palette of 16 base colors, or a color component (red, green or blue) of the adjacent pixel can be changed. The HAM screen mode makes it possible to use all possible 4096 colors, albeit with some restrictions on the adjacent color values.
Another unique selling point of the Amiga were its sound capabilities. It could mix 4 audio channels in hardware, and easily combined with graphics, animations and games. The Amiga has all kinds of interesting music productivity software, such as ProTracker, that I used a lot.
To make all these multimedia features possible, the Amiga has its own unique hardware architecture:
The above diagram provides a simplified view of the most important chips in the Amiga 500 and how they are connected:
- On the left, the CPU is shown: a Motorola 68000 that runs at approximately 7 MHz (the actual clock speeds differ somewhat on a PAL and NTSC display). The CPU is responsible for doing calculations and executing programs.
- On the right, the unique Amiga chips are shown. Each of them has a specific purpose:
- Denise (Display ENabler) is responsible for producing the RGB signal for the display, provides bitplane registers for storing graphics data, and is responsible for displaying sprites.
- Agnus (Address GeNerator UnitS) provides a blitter (that is responsible for quick transfers of data in chip memory, typically graphics data), and a copper: a programmable co-processor that is aligned with the video beam.
The copper makes all kinds of interesting graphical features possible, while keeping the CPU free for work. For example, the following screenshot of the game Trolls:
clearly contains more than 32 distinct colors. For example, the rainbow-like background provides a unique color on each scanline. The copper is used in such a way that the value of the background color register is changed on each scanline, while the screen is drawn.
The copper also makes it possible to switch between screen modes (low resolution, high resolution) on the same physical display, such as in the Workbench:
As can be seen in the above screenshot, the upper part of the screen shows Deluxe Paint in low-res mode with its own unique set of colors, while the lower part shows the workbench in high resolution mode (with a different color palette). The copper can change the display properties while the screen is rendered, while keeping the CPU free to do work.
-
Paula is a multi-functional chip that provides sound support, such as processing sample data from memory and mixing 4 audio channels. Because it does mixing in hardware, the CPU is still free to do work.
It also controls the disk drive, serial port, mouse and joysticks.
- All the chips in the above diagram require access to memory. Chip RAM is memory that is shared between all chips. As a consequence, they share the same memory bus.
A shared bus imposes speed restrictions -- on even clock cycles the CPU can access chip memory, while on the uneven cycles the chips have memory access.
Many Amiga programs are optimized in such a way that all CPU's memory access operations are at even clock cycles as much as possible. When the CPU needs to access memory on uneven clock cycles, it is forced to wait, losing execution speed. - An Amiga can also be extended with Fast RAM that does not suffer from any speed limitations. Fast RAM is on a different memory bus that can only be accessed by the CPU and not by any of the chips.
(As a sidenote: there is also Slow RAM that is not shown in the diagram. It falls in between chip and fast RAM. Slow RAM is memory that is exclusive to the CPU, but cannot be used on uneven clock cycles).
Compared to other computer architectures used at the same time, such as the PC, 7 MHz of CPU clock speed does not sound all that impressive, but the combination of all these autonomous chips working together is what makes many incredible multimedia properties possible.
My Amiga 500 specs
When my parents bought my Commodore Amiga 500 machine in 1992, it still had the original chipset and 512 KiB of Chip RAM. The only peripherals were an external 3.5-inch floppy drive and a kickstart switcher allowing me switch between Kickstart 1.3 and 2.0. (The kickstart are portions of the Amiga operating system residing in the ROM).
Some time later, the Agnus and Denise chips were upgraded (we moved from the Original Chipset to the Enchanced Chipset), extending the amount of chip RAM to 1 MiB and making it possible to use super high resolution screen modes.
At some point, we bought a KCS PowerPC board making it possible to emulate a PC and run MS-DOS applications. Although the product calls itself an emulator, it is also provides a board that extends the hardware with a number of interesting features:
- A 10 MHz NEC V30 CPU that is pin and instruction-compatible with an Intel 8086/8088 CPU. Moreover, it implements some 80186 instructions, some of its own instructions, and is between 10-30% faster.
- 1 MiB of RAM that can be used by the NEC V30 CPU for conventional and upper memory. In addition, the board's memory can also be used by the Amiga as additional chip RAM, fast RAM and as a RAM disk.
- A clock (powered by a battery) so that you do not have reconfigure the date and time on startup. This PC clock can also be used in Amiga mode.
Eventually, we also obtained a hard drive. The Amiga 500 does not include any hard drive, nor has it an internal hard drive connector.
Nonetheless, it can be extended through the Zorro expansion slot with an extension board. We obtained this extension board: MacroSystem evolution providing a SCSI connector, a whopping 8 MiB of fast RAM and an additional floppy drive connector. To the SCSI connector, a 120 MiB Maxtor 7120SR hard-drive was attached.
Installing new and replacement peripherals
In this section, I will describe my replacement peripherals and what I did to make them work.
RCB to SCART cable
As explained in the introduction, I no longer have a monitor and the Genlock device is broken, only making it possible to have a black and white display.
Fortunately, all kinds of replacement options seem to be available to connect an Amiga to a more modern display.
I have ordered an RGB to SCART cable. It can be attached to the RGB and audio output of the Amiga and to the SCART input on my LCD TV.
GoTek floppy emulator
Another problem is that the secondary floppy drive is broken and could not be repaired.
Even if I could find a suitable replacement drive, floppy disks are very difficult media to use for data exchange these days.
Even with an old PC that still has an internal floppy drive (capable of reading both high and double density floppy disks), exchanging information remains difficult -- due to the limitations of the PC floppy controller, a PC is incapable of reading Amiga disks, but an Amiga can read and write to PC floppy disks. A PC formatted floppy disk has less storage capacity than an Amiga formatted disk.
There is also an interesting alternative to a real floppy drive: the GoTek floppy emulator.
The GoTek floppy emulator works with disk image files stored on a USB memory stick. The numeric digit on the display indicates which disk image is currently inserted into the drive. With the rotating switch you can switch between disk images. It operates at the same speed as a real disk drive and produces similar sounds.
Booting from floppy disk 0 starts a program that allows you to configure disk images for the remaining numeric entries:
The GoTek floppy emulator can act both as a replacement for the internal floppy drive as well as an external floppy drive and uses the same connectors.
I have decided to buy an external model, because the internal floppy drive still works and I want to keep the machine as close to the original as possible. I can turn the GoTek floppy drive into the primary disk drive, by using the DF0 switch on the right side of the Amiga case.
Because all disk images are stored on a FAT filesystem-formatted USB stick, makes exchanging information with a PC much easier. I can transfer the same disk files that I can use in the Amiga emulator to the USB memory stick on my PC and then natively use them on a real Amiga.
SCSI2SD
As explained earlier, the 29-year old SCSI hard drive connected to the expansion board is showing all kinds of age-related problems. Although I could search for a compatible second-hand hard drive that was built in the same era, it is probably not going to last very long either.
Fortunately, for retro-computing purposes, an interesting replacement device was developed: the SCSI2SD, that can be used as drop-in replacement for a SCSI hard drive and other kinds of SCSI devices.
This device can be attached to the same SCSI and power connector cables that the old hard drive uses. As the name implies, its major difference is that is uses a (modern) SD-card for storage.
The left picture (shown above) shows the interior of the MacroSystem evolution board's case with the original Maxtor hard drive attached. On the right, I have replaced the hard drive with a SCSI2SD board (that uses a 16 GiB SD-card for storage).
Another nice property of the SCSI2SD is that an SD card offers much more storage capacity. The smallest SD card that I could buy offers 16 GiB of storage, which is a substantially more than the 120 MiB that the old Maxtor hard drive from 1992 used to offer.
Unfortunately, the designers of the original Amiga operating system did not forsee that people would use devices with so much storage capacity. From a technical point of view, AmigaOS versions 3.1 and older are incapable of addressing more than 4 GiB of storage per device.
In addition to the operating system's storage addressing limit, I discovered that there is another limit -- the SCSI controller on the MacroSystem evolution extension board is unable to address more than 1 GiB of storage space per SCSI device. Trying to format a partition beyond this 1 GiB boundary results in a "DOS disk not found" error. This limit does not seem to be documented anywhere in the MacroSystem evolution manual.
To cope with these limitations, the SCSI2SD device can be configured in such a way that it stays within the boundaries of the operating system. To do this, it needs to be connected to a PC with a micro USB cable and configured with the scsi2sd-util tool.
After many rounds of trial and error, I ended up using the following settings:
- Enable SCSI terminator (V5.1 only): on
- SCSI Host Speed: Normal
- Startup Delay (seconds): 0
- SCSI Selection Delay: 255
- Enable Parity: on
- Enable Unit Attention: off
- Enable SCSI2 Mode: on
- Disable glitch filter: off
- Enable disk cache (experimental): off
- Enable SCSI Disconnect: off
- Respond to short SCSI selection pulses: on
- Map LUNS to SCSI IDs: off
Furthermore, the SCSI2SD allows you to configure multiple SCSI devices and put restrictions on how much storage from the SD card can be used per device.
I have configured one SCSI device (representing a 1 GiB hard drive) with the following settings:
- Enable SCSI Target: on
- SCSI ID: 0
- Device Type: Hard Drive
- Quirks Mode: None
- SD card start sector: 0
- Sector size (bytes): 512
- Sector count: leave it alone
- Device size: 1 GB
I left the Vendor, ProductID, Revision and Serial Number values untouched. The Sector count is derived automatically from the start sector and device size.
Before using the SD card, I recommend to erase it first. Strictly speaking, this is not required, but I have learned in a very painful way that DiskSalv, a tool that is frequently used to fix corrupted Amiga file systems, may get confused if there are traces of a previous filesystem left behind. As a result, it may incorrectly treat files as invalid file references causing further corruption.
On Linux, I can clear the memory of the SD card with the following command (/dev/sdb refers to the device file of my SD-card reader):
$ dd if=/dev/zero of=/dev/sdb bs=1M status=progress
After clearing the SD card, I can insert it into the SCSI2SD device, do the partitioning and perform the installation of the Workbench. This process turns out to be more tricky than I thought -- the MacroSystem evolution board seems to only include a manual that is in German, requiring me to brush up my German reading skills.
The first step is to use the HDToolBox tool (included with the Amiga Workbench 2.1 installation disk) to detect the hard disk.
(As a sidenote: check if the SCSI cable is properly attached to both the SCSI2SD device, as well as the board. In my first attempt, the firmware was able to detect that there was a SCSI device with LUN 0, but it could not detect that it was a hard drive. After many rounds of trial and error, I discovered that the SCSI cable was not properly attached to the extension board!).
By default, HDToolBox works with the standard SCSI driver bundled with the Amiga operating system (scsi.device) which is not compatible with the SCSI controller on the MacroSystem Evolution board.
To use the correct driver, I had to configure HDToolBox to use a different driver, by opening a shell session and running the following command-line instructions:
Install2.1:HDTools HDToolBox evolution.device
In the above code fragment, I pass the driver name: evolution.device as a command-line parameter to HDToolBox.
With the above configuration setting, the SCSI2SD device gets detected by HDToolBox:
I did the partitioning of my SD-card hard drive as follows:
| Partition Device Name | Capacity | Bootable |
|---|---|---|
| DH0 | 100 MiB | yes |
| KCS | 100 MiB | no |
| DH1 | 400 MiB | no |
| DH2 | 400 MiB | no |
I did not change any advanced file system settings. I have configured all partitions to use mask: 0xfffffe and max transfer: 0xffffff.
Beyond creating partitions, there was another tricky configuration aspect I had to take into account -- I had to reserve the second partition (the KCS partition) as a hard drive for the KCS PowerPC emulator.
In my first partitioning attempt, I configured the KCS partition as the last partition, but that seems to cause problems when I start the KCS PowerPC emulator, typically resulting in a very slow startup followed by a system crash.
It appears that this problem is a caused by a memory addressing problem. Putting the KCS partition under the 200 MiB limit seems to fix the problem. Since most addressing boundaries are power of 2 values, my guess is that the KCS PowerPC emulator expects a hard drive partition to reside below the 256 MiB limit.
After creating the partitions and rebooting the machine, I can format them. For some unknown reason, a regular format does not seem to work, so I ended up doing a quick format instead.
Finally, I can install the workbench on the DH0: partition by running the Workbench installer (that resides in the: Install2.1 folder on the installation disk):
Null modem cable
The GoTek floppy drive and SCSI2SD already make it much easier to exchange data with my Amiga, but they are still somewhat impractical for exchanging small files, such as Protracker modules or software packages (in LhA format) downloaded from Aminet.
I have also bought a good old-fashioned null modem cable that can be used to link two computers through their serial ports. Modern computers no longer have a RS-232 serial port, but you can still use an USB to RS-232 converter that indirectly makes it possible to link up with a USB connection.
To link up, the serial port settings on both ends need to be the same and the baud rate should not be to high. I have configured the following settings on my Amiga (configured with the SYS:Prefs/Serial preferences program):
- Baud rate: 19,200
- Input buffer size: 512
- Handshaking: RTS/CTS
- Parity: None
- Bits/Char: 8
- Stop Bits: 1
With a terminal client, such as NComm, I can make a terminal connection to my Linux machine. By installing lrzsz on my Linux machine, I can exchange files by using the Zmodem protocol.
There are a variety of ways to link my Amiga with a Linux PC. A quick and easy way to exchange files, is by starting picocom on the Linux machine with the following parameters:
$ picocom --baud 19200 \ --flow h \ --parity n \ --databits 8 \ --stopbits 1 \ /dev/ttyUSB0
After starting Picocom, I can download files from my Linux PC by selecting: Transfer -> Download in the NComm menu. This action opens a file dialog on my Linux machine that allows me to pick the files that I want to download.
Similarly, I can upload files to my Linux machine by selecting Transfer -> Upload. On my Linux machine, a file dialog appears that allows me to pick the target directory where the uploaded files need to be stored.
In addition to simple file exchange, I can also expose a Linux terminal over a serial port and use my Amiga to remotely provide command-line instructions:
$ agetty --flow-control ttyUSB0 19200
To keep the terminal screen formatted nicely (e.g. a fixed number of rows and columns) I should run the following command in the terminal session:
stty rows 48 cols 80
By using NComm's upload function, I can transfer files to the current working directory.
Downloading a file from my Linux PC can be done by running the sz command:
$ sz mod.cool
The above command allows me to download the ProTracker module file: mod.cool from the current working directory.
It is also possible to remotely administer an Amiga machine from my Linux machine. Running the following command starts a shell session exposed over the serial port:
> NewShell AUX:
With a terminal client on my Linux machine, such as Minicom, I can run Amiga shell instructions remotely:
$ minicom -b 19200 -D /dev/ttyUSB0
showing me the following output:
Usage
All these new hardware peripherals open up all kinds of new interesting possibilities.
Using the SD card in FS-UAE
For example, I can detach the SD card from the SCSI2SD device, put it in my PC, and then use the hard drive in the emulator (both FS-UAE and WinUAE seem to work).
By giving the card reader's device file public permissions:
$ chmod 666 /dev/sdb
FS-UAE, that runs as an ordinary user, should be able to access it. By configuring a hard drive that refers to the device file:
hard_drive_0 = /dev/sdb
we have configured FS-UAE to use the SD card as a virtual hard drive (allowing me to use the exact same installation):
An advantage of using the SD card in the emulator is that we can perform installations of software packages much faster. I can temporarily boost the emulator's execution and disk drive speed, saving me quite a bit of installation time.
I can also more conveniently transfer large files from my host system to the SD card. For example, I can create a temp folder and expose it in FS-UAE as a secondary virtual hard drive:
hard_drive_1 = /home/sander/temp hard_drive_1_label = temp
and then copy all files from the temp: drive to the SD card:
Using the KCS PowerPC board with the new peripherals
The GoTek floppy emulator and the SCSI2SD device can also be used in the KCS PowerPC board emulator.
In addition to Amiga floppy disks, the GoTek floppy emulator can also be used for emulating double density PC disks. The only inconvenience is that it is impossible to format an empty disk on the Amiga for a PC with CrossDOS.
However, on my Linux machine, it is possible to create an empty 720 KiB disk image, format it as a DOS disk, and put the image file on the USB stick:
$ dd if=/dev/zero of=./mypcdisk.img bs=1k count=720 $ mkdosfs -n mydisk ./mypcdisk.img
The KCS PowerPC emulator also makes it possible to use Amiga's serial and parallel ports. As a result, I can also transfer files from my Linux PC by using a PC terminal client, such as Telix:
To connect to my Linux PC, I am using almost the same serial port settings as in the Workbench preferences. The only limitation is that I need to lower my baud rate -- it seems that Telix no longer works reliably for baud rates higher than 9600 bits per second.
The KCS PowerPC board is a very capable PC emulator. Some PC aspects are handled by real hardware, so that there is no speed loss -- the board provides a real 8086/8088 compatible CPU and 1 MiB of memory.
It also provides its own implementation of a system BIOS and VGA BIOS. As a result, text-mode DOS applications work as well as their native XT-PC counterparts, sometimes even slightly better.
One particular aspect that is fully emulated in software is CGA/EGA/VGA graphics. As I have explained in a blog written several years ago, the Amiga uses bitplane encoding for graphics whereas PC hardware uses chunky graphics. To allow graphics to be displayed, the data needs to be translated into planar graphics format, making graphics rendering very slow.
For example, it is possible to run Microsoft Windows 3.0 (in real mode) in the emulator, but the graphics are rendered very very slowly:
Interestingly enough, the game: Commander Keen seems to work at an acceptable speed:
I think Commander Keen runs so fast in the emulator (despite its slow graphics emulation), because of the adaptive tile refresh technique (updating the screen by only redrawing the necessary parts).
File reading problems and crashes
Although all these replacement peripherals are nice, such as the SCSI2SD, I was also running into a very annoying recurring problem.
I have noticed that after using the SCSI2SD for a while, sometimes a file may get incorrectly read.
Incorrectly read files lead to all kinds of interesting problems. For example, unpacking an LhA or Zip archive from the hard drive may sometimes result in one or more CRC errors. I have also noticed subtle screen and audio glitches while playing games stored on the SD card.
A really annoying problem is when an executable is incorrectly read -- this typically results in program failure crashes with error codes 8000 0003 or 8000 0004. The former error is caused by executing a wrong CPU instruction.
These read errors do not seem to happen all the time. For example, reading a previously incorrectly read file may actually open it successfully, so it appears that files are correctly written to disk.
After some investigation and comparing my SD card configuration with the old SCSI hard drive, I have noticed that the read speeds were a bit poor. SysInfo shows me a read speed of roughly 698 KiB per second:
By studying the MacroSystem Evolution manual (in German) and comparing the configuration with the Workbench installation on the old hard drive, I discovered that there is a burst mode option that can boost read performance.
To enable burst mode, I need to copy the Evolution utilities from the MacroSystem evolution driver disk to my hard drive (e.g. by copying DF0:Evolution3 to DH0:Programs/Evolution3). and add the following command-line instruction to S:User-Startup:
DH0:Programs/Evolution3/Utilities/HDParms 0 NOCHANGE NOFORMAT NOCACHE BURST
Resulting in read speeds that are roughly 30% faster:
Unfortunately, faster read speeds also seem to dramatically increase the likelyhood on read errors making my system quite unreliable.
I am still not completely sure what is causing these incorrect reads, but from my experiments I know that read speeds definitely have something to do with it. Restoring the configuration to no longer use burst mode (and slower reads), seems to make my system much more stable.
I also learned that these read problems are very similar to problems reported about a wrong MaxTransfer value. According to this page, setting it to 0x1fe00 should be a safe value. I tried adjusting the MaxTransfer value, but it does not seem to change anything.
Although my system seems to be stable enough after making these modifications, I would still like to expand my knowledge about this subject so that I can fully explain what is going on.
UPDATE 2024: I have discovered that these unstable reads may have something to do with power -- when I attach the SCSI2SD board to a power socket through the USB-C socket (while also keeping the SCSI power cable attached), it seems that the board is much more stable and I can use it reliably for hours. Maybe there is something wrong with the SCSI power connector or the power for SCSI termination.
Conclusion
It took me several months to figure out all these details, but with my replacement peripherals, my Commodore Amiga 500 works great again. The machine is more than 29 years old and I can still run all applications and games that I used to work with in the mid 1990s and more. Furthermore, data exchange with my Linux PC has become much easier.
Back in the early 90s, I did not have the luxury to download software and information from Internet.
I also learned many new things about terminal connections. It seems that Linux (because of its UNIX heritage) has all kinds of nice facilities to expose itself as a terminal server.
After visiting the home computer museum, I became more motivated to preserve my Amiga 500 in the best possible way. It seems that as of today, there are still replacement parts for sale and many things can be repaired.
My recommendation is that if you still own a classic machine, do not just throw it away. You may regret it later.
Future work
Aside from finding a proper explanation for the file reading problems, I am still searching for a real replacement floppy drive. Moreover, I still need to investigate whether the Genlock device can be repaired.
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