Archive for the 'Micro Computers' Category


Commodore 64 Microcomputer

Today I’m looking at a popular micro computer from the early 80’s the Commodore 64 or C64 for short. It was released in 1982 after machines like the Apple II, TRS 80 and CBM PET machines started a market for computers in the home. They became one of the most popular computers of all time selling more units than any other single model.

Commodore were able to under cut many of their competitors because they owned the chip maker MOS Technologies. This meant they could use chips like the 6502 at a lower cost and could include custom chips like the VIC II and SID with out paying as much for the chips. Other machines such as the ZX Spectrum, Apple II, TRS 80 and Atari 800 either had to use less capable of the shelf parts or pay more for chips with the same capability.



I have a C64c which is the later cost reduced version of the Commodore. You can see that it has a more modern chassis and now has a ventilation grill at the top. It does have the same keyboard, albeit in a different colour. The main changes are mostly on the main board with a reduced chip count and lower voltage to improve the reliability and cost of the machine.



I bought the Datasette for the machine first as many software titles came on audio cassette. I’ve found with the help of software and a tape player I was able to convert C64 images to audio tape for playing on the real machine. It takes ages for software to load but it allows you to play tape images you can get online. Recently I bought the SD2IEC pictured in the right of the photo. It allows you to load programs from an SD card. It connects to the floppy drive port and emulates it right down to the slow load times.



Here are the controllers I bought for my machine, a set of Commodore paddles as some games require them and two Competition Pro joysticks. The joysticks are good, although possibly in need of cleaning the contacts. They make a satisfying click when you press a button or move the stick. The paddles are reasonably accurate and aren’t suffering from jitter, a common problem for older paddles.



I was able to amass a good collection of cartridges for my system pretty quickly. I bought most of them in packs of 4 or 5 at a relatively good price. It is usually fairly easy to get games at a good price because they were fairly common. Cartridges less so as many games were distributed by tape or disk. I quite like having cartridges as they have less problems with media damage, especially if using a mask ROM. Floppy disks probably suffer the most from media damage and from damage to the floppy drives. Audio tape also suffers loss over time, but it usually isn’t as severe and many old games still load. I’d still archive them digitally but you’re less likely to have problems with them.

I didn’t own a C64 back in the day, so it was interesting getting one and trying out the software. When I first tried out the tape drive it did take a painfully long time to load anything but it was worth it once the game loaded and you certainly made the most of each session as it would take ages to load something else! The cartridge games have been fantastic because of the short loading times. I particularly like Wizard of Wor and Zenji but all the cartridge games are quite good.

Because the C64 was so common all the software, accessories and the machine itself are easy to find and relatively cheap. Finding software for devices like the SD2IEC is also easy because to the large internet community still supporting the machine. In short I highly recommend it for new collectors as an easy way to get your feet wet using old machines.


Amstrad NC100 Laptop

Amstrad NC100

Amstrad NC100

Today I’m looking at one of the micro computers in my small collection, the Amstrad NC100 laptop. It was released back in 1992 arguably after Z80 machines were no longer relevant. However because it was small and inexpensive it sold quite well because PC laptops of the time were quite expensive.

The machine is faster than other Z80 portables of similar vintage running at 6Mhz! It has 64K of battery backed up RAM which can’t really store all that many programs or data. Fortunately the built in software is quite good for basic tasks such as word processing and managing a diary and address book. The built in RAM can be expanded with PCMCIA SRAM cards and I am fortunate enough to have the maximum upgrade in the form of a 1Mb card, now if only I could get a 2325 battery to replace the dead one in it.

Expansion Card

Expansion Card

You can transfer files to and from the machine using XModem transfer over serial. The machine was frequently used by journalists in the field to write documents and be able to submit their reports via modem. It’s also useful for transferring programs to the machine or to PC for backup. There is a serial terminal built into the ROM so it could be used to access BBS services or act as a terminal for a larger machine.

The machine has BBC Basic on it, which was one of the better interpreters of the micro computer era. There are a number of statements that don’t work however, but it’s all fairly well documented which is fortunately available online. A number of people have developed different games and applications for it. The interpreter is of course not as fast as machine code, so if you know Z80 assembler that’s a better way to code for the machine.

It uses standard AA batteries and has quite a generous running time on a fresh set. The display is a simple 80×4 character text LCD with graphics capability, so the screen doesn’t use up tones of juice. It can be a bit hard to read depending on the light, but there is a contrast control to help make it more readable. The keyboard is a bit mushy, but quite usable and certainly better than some of the really cheap keyboards around.

I found the Amstrad NC100 a fascinating machine to use. It’s built in ROM makes creating documents and basic organiser functions quite easy, and the program-ability of BBC Basic makes it quite versatile. It is however quite limited in many aspects such as the display and amount of memory, but for the time it was one of the best portables around. I got mine of Ebay some time ago, and I frequently use it as an easy to store and use serial terminal for my bigger Sun machines. I would like to own a NC200 for the larger screen and floppy, but those appear less frequently at a price I’m willing to pay.

Some pages that would be interesting to owners of the Amstrad NC100/NC200

Tim’s Amstrad User Site

Tear-down on EEVBlog for repair

Finally as it is that time of year, I’d like to wish everyone a happy holiday season, which-ever holiday it is that you celebrate.


Old Computer Books

Unfortunately I’ve been rather sick the last two weeks with glandular fever, and so I haven’t really been able to post anything. Fortunately I’m starting to feel much better, but I find myself unprepared!

So today I’m being lazy and just showing some photos of some books I recently acquired.

The University I work for recently had a big clean out in the main IT building. They were offering a number of their old computer books for free to both staff and students alike. I found some interesting books in amongst the more modern ones. There were a number of books for programming in Pascal and a set of manuals for Borland Turbo Pascal 6.0. This was good for me as I have the old Turbo Pascal 6.0 software (and have been using it for some time) and having a manual with more detail than the online help has proven to be quite useful. I also found a Fortran book (by Microsoft) which may be interesting reading as I’ve never programmed in Fortran before.

There are also a number of old books documenting old DEC hardware such as the PDP 11 processor and a few with more system specific information. I’ve never owned or used such hardware as it was a bit before my time in computing, so reading these books should be interesting. I had heard programming for the PDP 11 was interesting and sometimes frustrating.

Finally I got three books about micro computers of the early eighties and a CP/M guide book. Again most of the machines talked about in the books are before my time, so this will most likely also be some interesting reading!

Some photos of the books follow.

Turbo Pascal Manuals

Turbo Pascal Manuals

DEC books

DEC books

Programming Books

Programming Books

Mystery Machine!

Mystery Machine!

Microcomputing handbooks!

Microcomputing handbooks!


Micro Computer Comparison – Part 3

I’ve been investigating the differences between the ZX Spectrum and the Commodore 64, and I’ve found previously that the ZX Spectrum is fortunate to have a slightly better processor and more memory bandwidth. You can read about those parts here and here.

Today in the final part I’ll be looking at the hardware external to the processor and memory. In the case of the Spectrum that will be the ULA, and the VIC and SID chips within the C64. This is one of the areas in which these two machines are the most different and also what makes a big difference to the user experience of both machines.

Continue reading ‘Micro Computer Comparison – Part 3’


Micro Computer Comparison – Part 2

In the first part of this series I compared the memory bandwidth of the Commodore 64 (know as the C64 for short) and the ZX Spectrum. If you haven’t read the first part you will find it here.

Today I will compare the two microprocessors used by the systems, that is the Moschip 6502 and the Zilog Z80 processors. In the last part I found that memory bandwidth wise, both processors have roughly the same ability at the speeds used and that the difference in memory performance was up to the physical architecture of the systems in which they are operating in. To understand how much work each processor is capable of doing I will be looking at the impact of the internal structure and instruction sets.

This will most likely be another very long post!

Continue reading ‘Micro Computer Comparison – Part 2’


Micro Computer comparison – Part 1

Recently I ran into the rantings of fan boys about the Commodore 64 and the Sinclair ZX Spectrum. I wasn’t very satisfied with the arguments of either side, as they were both filled with bias and ignorance, just like pretty much any flaming on the internet. One C64 fan even claimed the 6502 was faster than the IBM PC (an 8088 @ 4.77Mhz). So I decided to do some reading of data sheets myself to get a more balanced and accurate view of the real potential of the machines. Just to clarify, whilst I own one of each kind of machine, I didn’t have either as a kid and thus don’t have childhood memories to bias my opinion one way of the other.

This is an incredibly long article so read on if you dare!

Continue reading ‘Micro Computer comparison – Part 1’


ZX Spectrum repair

Image clearer - but corrupted

Image clearer – but corrupted

A couple of weeks ago I went to use my spectrum. I modded the circuit to output composite video instead of RF to make connecting it easier, and to clean up the displayed image. The mod was a great success but I discovered bad video ram was causing my machine to not boot up and only display garbage.The garbage had a pattern to it that looked to me like a single bit in the video ram was faulty. Two chips in the video memory were already socketed, probably from a previous repair, and after swapping them I suspected one of them was faulty. The video ram on a spectrum is organised into a bank of 8 4116 memory chips, each one responsible for one bit out of a byte word of video RAM. The original chips are increasingly difficult to find, this makes it difficult to repair them without buying a sacrificial machine to take parts from.



I searched high and low for some replacement chips and found some AMD equivalent plug-in replacements that I thought would do the job. They are AM9016FPC chips that are 150μs in speed. They are pretty much identical to the original 4116 chips that were originally in the spectrum, so I bought a number of them as spares, in fact enough to replace the entire video memory bank if required.

It's working!

It’s working!

Just this weekend gone the chips arrived and I had time to try them out. So  I dismantled my spectrum, removed one of the Texas Instrument chips in the sockets and replaced it with the new AMD chip. It wasn’t long before I had my spectrum back in working order! The fact that one of the chips had failed concerned me, so I decided to write a simple basic program to test all the memory I could in a simple way, to ensure that no others had died.

So after reading a memory map for the spectrum I banged out a short program to do some simple tests of the ram. Here is some of the code.

Test Program

Test Program

10 LET l=0
20 LET s=25001
30 LET e=65535
40 FOR i=s to e
50 LET c=INT (i/255)
60 IF c=l THEN GO TO 80
65 LET l=c
70 PRINT AT 1,1;"mem addr ";i
80 POKE i,0
90 IF PEEK i=0 THEN GO TO 110
100 PRINT "error ";i;" ";PEEK i
110 POKE i,255
120 IF PEEK i=255 THEN GO TO 140
130 PRINT "error 2 ";i;" ";PEEK i
140 NEXT i

You can adjust this code to test other areas of memory by changing the s and e variables. To run this code you should type CLEAR 25000 first so none of the memory used for basic is overwritten. It tests the upper portion of the contended memory and all of the 32K bank. Progress is printed every 255 bytes. It’s not a very thorough test of the memory as it doesn’t check for cross-links or other memory faults. It could of course be adapted for that, but would take significantly longer to perform the test, unless programmed in assembly.

I ran this on my spectrum and everything appeared fine, I even left the machine on over night and tested again in the morning, so I’m satisfied that the memory is now in good condition.

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