Over on YouTube, user @ms-ex8em pointed me to the November 1988 issue of Dragon User magazine and a little program that helps you type in machine code program listings accurately.
I recall spending countless hours typing in machine code programs from Rainbow Magazine “back in the day”. These programs would be a few lines of BASIC followed by dozens or hundreds of lines of DATA statements with numbers that represented the bytes of the machine language program.
One such program I specifically remember was Zonx, a cool game with background sound effects. It appeared in the October 1985 issue. You can see what I typed in by checking out page 65. And page 67. And page 68. And page 70. And page 71.
And, somehow, I managed to type all of those numbers in and get a working program!
Rainbow did have one thing to help — and perhaps I used it. They had a small program you could run, then as you typed in your program listing, when you got to specific lines you could hit a key and it would spit out a number (a checksum?) and see if it matched their numbers:
Above, you can see what number should be expected when you got to line 150, 310, 440 and so on. After all the code was typed in, the final number it should give you was 238. This required you to type in the program exactly as it appeared including spaces and any other formatting.
It was not optimal, but it was better than nothing.
I also saw some BASIC loaders for machine language that took matters into their own hands, and had built-in checksum values in their DATA statements. For example, they would put a specific amount of numbers on each line, with a checksum of those numbers at the end. Say that amount of numbers was five. A line might look like this:
1500 DATA 1,1,1,1,1,5
In that silly example, you could add up ever five bytes you READ and then compare that to the 6th byte in each line. If it matched, continue. If it didn’t, the loader code would print some kind of message, hopefully indicating what line had a mismatch.
I always appreciated program listings that did that.
Now, back to @ms-ex8em… The code they shared was quite different. You can find it on page 12:
I will do my best to type it in here, accurately.
10 REM HEX LOADER
20 CLEAR 200,31599
30 INPUT"START";ST
40 INPUT"END";ED
50 FOR J=ST to ED STEP 8
60 PRINT USING"##### : ";J;
70 INPUT A$
80 CS=0
90 FOR K=1 TO LEN(A$)
100 CS=CS+K*VAL("&H"+MID$(A$,K,1))
110 NEXT K
120 INPUT" = ";C
130 IF S<>CS THEN PRINT"CHECKSUM ERROR-TRY AGAIN":SOUND 1,1:GOTO 60
140 FOR K=0 TO 7
150 POKE J+K,VAL("&H"+MID$(K*2+1,2)
160 NEXT K,JFirst comment… There appears to be a mistake in line 150. It should have two closing parens at the end of the line.
Now let’s see what this code is doing.
In line 10, the CLEAR reserves memory (so BASIC cannot use it) starting at 31599. This is because the machine language program will load starting at 31600.
In line 40, the user is asked to input the start and end of the program they will be typing in.
In line 50, there is a FOR/NEXT that goes from start to end, stepping by 8. This tells me they expect 8 bytes of data on each line the user types in. The first input should be at 31600, then the next at 31608 and so on.
The user types in the 16 characters (representing 8 hex bytes) shown in the box below. They look like this:
31600 : 8E7B6F9F74Be012B = 1016In line 80, a checksum is calculated by adding up the value of each character typed in that line.
In line 120, the user is asked to type in the checksum shown in the data listing.
In line 130, if the number the user typed in (from the data listing) does not match value the program calculated, an error message prints and it goes back to let the user retype the entire line.
If it matched, line 140 will then go through those 16 characters and create the byte value for each of the 8 data bytes and POKE them into memory at the proper place.
This repeats until the end address is reached. At that point, some new assembly language program has been loaded into memory starting at 31600.
Pretty cool!
The problem @ms-ex8em was seeing with this program was a ?TM ERROR. That is most likely showing up in line 150 because of the missing paren. Fix that, then maybe the rest will work ;-)
I thought this was a neat way to enter in machine code. What other neat ways did you see back then?
Comment away…