The Six Million Dollar (approximately) question.

 

That question is, what RAM is in this machine ? We already know it is small.

This drives how you access it to generate video ; that I will deal with later on. At the moment I'm looking at money.

I was reading Brian Bagnall's book and it seemed that the purchase price of various Commodore devices  for the user seemed to be about 3 times the cost of actually making the things.

Likewise, I have a vague memory that the cost of a Spectrum was about £45 at the factory, so to speak, which fits in roughly with these sorts of costs.

This gives us a production cost for the LC3 of something like £12-15. Which isn't much.

The strip on the right is a price list of chips in "Electronics Today International" April 1983, taken from Watford Electronics' advert.  Obviously these are you and me prices and there's a saving for bulk purchase.

Looking at Mouser, if I buy a single Z80 it would cost me £7.11. If I bought 1,000 of them, it would cost me £4.03 per item. I'll use this as a price guide. 

Additionally we have a similar component cost (RAM + ROM + ULA + Z80 + bits) to a ZX81 as a kit, so it's going to be slightly less than that, though of course we have no keyboard or manual but a sheet like the calculators but we have to add on the costs of building the thing.

Main CPU

So we know a Z80 is needed. In the Watford advert, a Z80A is going for £3, so we'll cost that at £2.00

Uncommitted Logic Array

In the Wikipedia Gate Array article it says "Ferranti introduced in 1982 a computer-aided design tool for their uncommitted logic array (ULA) product called ULA Designer. Although costing £46,500 to acquire, this tool promised to deliver reduced costs of around £5,000 per design plus manufacturing costs of £1-2 per chip in high volumes,"

Probably volume wouldn't be initially that high, so maybe £2 for the ULA.

System ROM

Jim Drew wrote in a forum posts regarding this question " EEPROMs of this size didn't exist back in the early to mid 80's. We were all using EPROMs (UV erased) or OTP ROMs for production runs. OTP ROMs were 1/2 the price of EEPROMs, and masked ROMs were 1/2 the price of OTP ROMs."

If that's correct, and there's a ROM in it with the "Operating System" then that could be a 2716 (£2.25) or perhaps a 2764 (£4.25), giving again another £2 or so to the cost.

RAM Memory

There's various options for RAM, most of which are on the advert. All prices are those prices so halve them or thereabouts.

• 1k SRAM as 2x2114 = £1.90 (say £0.95)
• 2k SRAM as 1x6116 = £3.50 (say £1.75)
• 4k DRAM as 8x4027 = £15.20 (say £7.60)
• 16k DRAM as 8x4116 = £6.80 (say £3.40)

I did consider the option of using a 8k RAM chip but these didn't appear until mid 1984 and were also relatively expensive. 

One can discount the 4027s immediately. 

On paper the DRAM looks tempting, but the design is full of 'minimum RAM' statements. Obviously it's small compared to a 48k Spectrum, but given how much of that would be used for non-variant code and the console is meant to be driven by cartridges it's quite a reasonable chunk. 

That leaves us with 2 options. A pair of 2114s at £0.95, or a single 6116 at £1.75.

We already have, maybe £6 of ICs, plus the usual plugs, sockets, PCB, case and so on. Two choices, one double the other but costing double the price.

The next article will deal with this choice. 

Note: One option this does not consider is the "duff RAM chip" Spectrum design. I did find a note that states "They only appeared for a very short time around '80/'81" and the general consensus of opinion is this is a one-off.