If lack of speed is a problem, or you would like more memory, then this second processor may be what you're after!
This month I have had the good fortune to review an excellent new addon which will be of interest to all Electron owners... a complete 6502 second processor upgrade which will make the Electron operate at the speed of a BBC Micro, allow users to load ROM software (written for both the Beeb and the Electron) and also act as a tremendous memory boost for languages, BASIC included, as well as run the currently available "HI" versions of BASIC and VIEW.
Now this all seems too good to be true, especially as the whole caboodle comes to around £70 but you will have to build the project yourself. However, as one reader who has built the board himself said, "I did it in an afternoon, even though it did involve me buying a fine-tipped soldering iron for the job... you can imagine my joy and satisfaction when I discovered that it worked first time!"
So what do you get for your money?
The bare PCB costs £12 and the software to drive the E2P is £3.50. If you own a Plus 3, then, if you send a disc, and £2, you can load it from disc as a !BOOT file.... it is also possible to EPROM the software, but as the code is quite small, there really seems no point in it.
The PCB is about the size of a postcard and is designed to plug into the Plus 1 socket. If you don't however, have a Plus 1 (and they are now getting very cheap), you can send for details on how to connect a special cable from the Elk's edge connector to the PCB; you may even like to house the PCB in a special case, as I know some people have done.
The E2P will work for BBC Micros as well, and, like the edge connector information, a special sheet is available on request. The E2P's 6502 is a 2MHz version (the same as the Electron and the Model B) and not a 3MHz version, like the one in the Acorn second processors. There are still advantages for Beeb users, especially if you use the extra power for graphics programming, as the Beeb does speed up, since the E2P handles the graphics plotting and the Beeb handles the program and calculations.
There are full assembly instructions enclosed with the PCB kit, as well as a list of all the components needed to build the completed kit; in addition you could look at the June/July issues of Electronics Today for construction and a listing of the software, should you feel you want to save on the software.
Like the BBC Micro, the display memory can consume between 8K of user RAM (in Mode 6) and 20K (in Modes 0, 1 and 2); add to this the 3.5K for OS workspace, (even more if you fit an ADFS system), and 1.5K for exploded character fonts, and all this will leave you with some 3.25K for programming, which is a good deal less than an expanded ZX81!
The E2P has an onboard bank of 64K RAM, 30K of whcih can be used from BASIC or any ROM (or images of other languages); if you come across any HI versions like HI-BASIC or even HI-VIEW, then the language relocates itself to make some 40K available for language workspace. If, like me, you end up writing machine code for the Beeb/Electron, you can fit a massive 60K providing you use a ROM-based assembler/disassembler. It is also possible to use the Acorn 6502 development system in conjunction with the E2P, which is a good indication of the board's compatibility.
The software author has supplied me with benchmark timings, which for once are accurate; as you can see in Figure 1, the E2P puts the Electron on a par with the unexpanded Beeb and the memory gain is greater than that of a Shadow RAM system as well!
For those hardware minded amongst us, the E2P works in the same way as the "official" Acorn secnd processor, where one machine is termed the "Host" processor and the other is called the "Parasite". I don't think it needs too much imagination to sort out which one is which.
The E2P sticks closely to the ULA recommendations set out by Acorn, with 8 bi-directional registers at &FCE0 to &FCE7; seven of these bits are used by supporting software and &FCE5 is accessed by the OS during block data transfer (e.g. LOAD or SAVE). The 256 byte block is accessible to the Elk and can be used as a bidirectional register; it also acts as a ROM image pointer. Here the term "image" is used carefully as both disc "images" (or ROM files) and hardware "images", such as a copied bank from the Elk to the E2P, can exist.
The software, although not as flashy as the Acorn ROM software, with its *HELP message "TUBE parasite connected", as it is on my B+ 128K, I can say that once you have loaded it, you will find that you will not need to reload the software at all, as it resides in only one block of memory and that is very low down in the map.
If you look at Figure 1, you can see that the Electron handles most of the processing for display work, and also the OS calls and E2P software, leaving in the E2P, a huge chunk for user programs, sidewaysROMs and a copy of the OS; the combination makes the Elk run like greased lightning and has no speed loss even with larger programs.
There may be problems, however, when fitting other hardware devices, such as third party disc drives, ROM boards and so on. This is because hardware and software from other manufacturers will not allow the E2P to operate properly, which is not the E2P's fault; as the programming for these has not stuck to Acorn guidelines, their software will be directly at odds with Acorn approved products (which the E2P is not, but it does conform rigidly to their hardware demands).
Another use to which Elk owners may put the second processor is running ROM software. I need not go on about the insane wait people have had for Elk versions of the BBC programming languages COMAL, PROLOG, LISP, ISO-PASCAL, MICROTEXT PLUS and even TURTLE GRAPHICS. The E2P represents a significant saving of time and money when it comes to software development... all of the above, bar MICROTEXT PLUS, work properly (Mode 7 is not possible in the latter). Acornsoft's LOGO, ISO-PASCAL, and any future dual ROM languages, will need a hardware addon, like the Slogger ROMBOX, to hold both ROMs in the machine at the same time.
I am not very good with a soldering iron. This amazing realisation came just before Christmas when I left my hand with a nice scar across it. Nevertheless but I was able to construct a second processor board for myself in just under a day, and after a false start (stupid me putting a diode round the wrong way!) I had my E2P up and running. In fact part of this review was written with HI-VIEW and the E2P on the table. The next thing I shall attempt is a housing for the little beastie.
The board is double-sided, which means that circuit links with bits of wire need to be added first. I would also recommend a special spray to stop dry joints on the board. Whatever you do, when you buy the memory chips, do not put them in a plastic bag. I have destroyed chips before by stuffing them in a plastic bag. Make sure they are "plugged in" onto the black foamlike substance, and then wrap them in foil. Finger the chips as little as possible and, where possible, socket everything, as a desoldered board is a nuisance to fit new chips into... all of these suggestions are in the list of parts, in the kit's manual.
There are services that build projects like the E2P, if you don't mind paying the extra, but if you have ever resoldered anything in the Electron or BBC board, then you should have no difficulty with this one.