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By some coincidence I happened to stumble upon a week-old, unanswered question posted to Hacker News regarding a bug in Sinclair BASIC on a Timex Sinclair 1000 microcomputer. While I never owned a TS1000, the post attracted my interest. I've studied ZX81, an almost identical microcomputer, extensively when I was doing my research on Galaksija. It also reminded me of a now almost forgotten idea to write a post on some obscure BASIC bugs in Galaksija's ROM that I found mentioned in contemporary literature.

The question on Hacker News is about the cause of a bug where the computer, when attached to a printer, would print out certain floating point numbers incorrectly. The most famous example, mentioned in the Wikipedia article on Timex Sinclair 1000, is the printout of 0.00001. The BASIC statement:

LPRINT 0.00001

unexpectedly types out the following on paper:

0.0XYZ1

This bug occurs both on Timex Sinclair 1000 as well as on Sinclair ZX81, since both computers share the same ROM code. Only the first zero after the decimal point is printed correctly while the subsequent zeros seem to be replaced with random alphanumeric characters. The non-zero digit at the end is again printed correctly. Interestingly, this only happens when using the LPRINT (line-printer print) statement that makes a hard-copy of the output on paper using a printer. The similar PRINT statement that displays the output on the TV screen works correctly (you can try it out on JtyOne's Online Emulator).

The cause of the bug lies in the code that takes a numerical value in the internal format of the BASIC's floating point calculator and prints out individual characters. One particular part of the code determines the number of zeros after the decimal point and uses a loop to print them out:

;; PF-ZEROS
L16B2:  NEG                     ; Prepare number of zeros
        LD      B,A             ; to print in B.

        LD      A,$1B           ; Print out character '.'
        RST     10H             ; 

        LD      A,$1C           ; Prepare character '0' 
				; to print out in A.

;; PF-ZRO-LP
L16BA:  RST     10H             ; Call "print character" routine
        DJNZ    L16BA           ; and loop back B times.

(This assembly listing is taken from Geoff Wearmouth's disassembly. Comments are mine.)

The restart 10h takes a character code in register A and either prints it out on the screen or sends it to the printer. Restarts are a bit like simple system calls - they are an efficient way to call an often-used routine on the Z80 CPU. The problem lies in the fact that this restart doesn't preserve the contents of the A register. It does preserve the contents of register B and other main registers through the use of the EXX instruction and the shadow registers, however the original contents of A is lost after the call returns.

Since the code above doesn't reset the contents of the A register after each iteration, only the first zero after the decimal point is printed correctly. Subsequent zeros are replaced with whatever was junk left in the A register by the 10h restart code. Solution is to simply adjust the DJNZ instruction to loop back two bytes earlier, to the LD instruction, so that the character code is stored to A in each iteration. You can see this fix in Geoff's customized ZX81 ROM, or in Timex Sinclair 1500 ROM (see line 3835 in this diff between TS1500 and TS1000).

This exact same code is also used when displaying numbers on the TV screen, however in that case it works correctly. The reason is that when set to print to screen, printing character 0 via the 10h restart actually preserves the contents of register A. Looking at the disassembly I suspect that was simply a lucky coincidence and not a conscious decision by the programmer. Any code calling 10h doesn't know whether the printer or the screen is used, and hence must assume that A isn't preserved anyway.

Of course, I'm far from being the first person to write about this particular Sinclair bug. Why then does the post on Hacker News say that there's little information to be found about it? The Wikipedia article doesn't cite a reference for this bug either.

It turns out that during my search for the answer, the three most useful pages were no longer on-line. Paul Farrow's ZX resource centre, S. C. Agate's ZX81 ROMs page and Geoff Wearmouth's Sinclair ROM disassemblies are wonderful historical resources that must have taken a lot of love and effort to put together. Sadly, they are now only accessible through the snapshots on the Internet Archive's Wayback Machine. If I wouldn't know about them beforehand, I probably wouldn't find them now. For the last one you even need to know what particular time range to look at on Archive.org, since the domain was taken over by squatters and recent snapshots only show ads (incidentally, this is also the reason why I'm re-hosting some of its former content).

I feel like we can still learn a lot from these early home computers and I'm happy that questions about them still pop-up in various forums. This LPRINT bug seems to be a case of a faulty generalization. It's a well known type of a mistake where the programmer wrongly generalizes an assumption (10h preserves A) that is in fact only true in a special case (displaying character on screen). History tends to repeat itself and I believe that many of the blunders in modern software wouldn't happen if software developers would be more aware of the history of their trade.

It's sad that these old devices are disappearing and that primary literature sources about them are hard to find, but I find it even more concerning that now it seems also these secondary sources are slowly fading out from general accessibility on the web.