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UnderColor, Volume 1, Number 1, December 10, 1984
- Title: Memory Test
- Author: Stephen P. Allen
- Synopsis: Do you really have 64k?
- Page Scans: Link
Memory prices the way they are now make it easy for a lot of people to upgrade their computer to a full 64K. It's a great feeling of satisfaction—"Aah, now I've got more power!"
"Wait a minute, how do I know I've really got 64K? Radio Shack says I've got to get a disk drive and OS·9 to use it. I don’t really know if I even have it! I wish there was some way to test."
ls that you? Run this program.
RAMCHECK will give your computer memory a proper workout. lf your memory chips extend all the way to 64K, you'll return from this program running Basic in RAM. And instead of ok, y0u’lI see co as the prompt. If your RAMs flunk the test, RAMCHECK will tell you where your memory effectively ends.
The assembly language routine will alter the contents of all memory above 16K. The first job of the Basic program is to move the 6809’s stack out of the way (leaving room for the routine). Don't change the addresses in Lines 30 and 40! lf you move them higher there’s the good chance the routine or the stack will be over-written during the test.
After the necessary space is cleared, the routine is POKEd in and called with a USR function. The value of AR, returned by the assembly language subroutine, will be zero if you have 64K, or it will be the address where memory "ceased to exist." GIVABF, at $B4F4, will treat any number given it that's larger than 32767 as negative. That's why 65536 is added if AR<0.
Switching between ROM and RAM is done by the 6883, that 40-pin mystery responsible for so much behind-the-scenes footwork. In all those 40 pins there are no data lines-the 6883 is controlled by simply writing to one of its 32 memory addresses. What’s written doesn't matter; it can’t read data, only addresses. The addresses that matter to us here are $FFDE (select ROMs) and $FFDF (select equivalent RAMs).
The byte values used to test the RAMs are $55 and SAA. In binary these correspond to 01010101 and 10101010. The values were chosen because these are the most difficult for memory to store—a one next to a zero may "influence" that zero to leak up to one, and vice-versa. I say may influence; I haven't seen it happen yet. (end)
Program Listing. RAM Check (Basic) 10 ' RAMCHECK 20 ' 30 ' TESTS FOR 64K RAMS 40 ' IF YOU HAVE THEM YOU RETURN 50 ' IN ALL RAM MODE 60 ' ELSE END OF MEMORY IS 70 ' DISPLAYED 80 ' 90 CLS:PRINTSTRING$(32,42) 100 PRINT@75,"RAMCHECKER 110 CLEAR200,12200 120 AD=12202:DEFUSR0=AD 130 READ OP:IFOP=1000 THEN150 140 POKEAD,OP:AD=AD+1:GOTO130 150 PRINT"CHECKING NOW...":HR=USR0(0) 160 IFHR=0THEN190ELSEIFHR<0THENHR=HR+65536 170 PRINT"RAM TEST FAILS AT $"HEX$(HR) 180 CLEAR200, HR-1:END 190 PRINT"NOW IN RAM!":CLEAR200,&H7FFF:END 200 DATA 142,63,255,26,80,183,255 210 DATA 223,134,85,167,132,161,132 220 DATA 38,44,72,104,132,161,132 230 DATA 38,37,48,1,140,255,0 240 DATA 37,234,1&2,128,0,183,255 250 DATA 222,166,132,183,255,223,167 260 DATA 128,140,240,0,37,241,13& 270 DATA 71,198,79,253,171,238,79 280 DATA 95,126,180,244,183,255,222 290 DATA 31,16,126,180,244,1000 Program Listing. RAM Checker (Assembly Language) 00100 * RAMCHECKER 00110 00120 * Tests for 64K rams ERROR 003C 00130 * If you have them it copies LOOP1 0008 00140 * Basic to Ram and returns. LOOP2 0021 00150 * Else it tells where the Ram ends. START 0000 00160 00170 00180 0000 8E 3FFF 00190 START LDX #$3FFF *Point to start of 32K 0003 1A 50 00200 ORCC #$50 *Interrupts off 0005 B7 FFDF 00210 STA $FFDF *Select "A11 Ram" mode 00212 0008 86 55 00220 LOOP1 LDA #$55 *Binary 01010101 000A A7 84 00230 STA ,X *Store it 000C A1 84 00240 CMPA ,X *Did we get it back? 000E 26 2C 00250 BNE ERROR *if not, this is end of Ram 0010 48 00260 ASLA *binary 10101010 0011 68 84 00270 ASL ,X *Same to Ram 0013 A1 84 00280 CMPA ,X *Did the Ram follow? 0015 26 25 00290 BNE ERROR *if not, end of Ram 0017 30 01 00300 LEAX 1,X *Test OK, move up one 0019 8C FF00 00310 CMPX #$FF00 *Start of PIAs? 001C 25 EA 00320 BLO LOOP1 *not yet-- keep going 00330 00332 * TEST SUCCESSFUL: 00340 * COPY BASIC TO RAM 00350 001E 8E 8000 00360 LDX #$8000 *Start of Basic 0021 B7 FFDE 00370 LOOP2 STA $FFDE *Se1ect Roms 0024 A6 84 00380 LDA ,X *Grab a byte from Rom 0026 B7 FFDF 00390 STA $FFDF *Se1ect Ram mode 0029 A7 80 00400 STA ,X+ *Dup1icate byte in Ram 002B 8C F000 00410 CMPX #$F000 *End of Roms? 002E 25 F1 00420 BLO LOOP2 *Not yet 00430 00440 * CHANGE "OK" TO "GO" 00450 0030 86 47 00460 LDA #'G 0032 C6 4F 00470 LDB #’O 0034 FD ABEE 00480 STD $ABEE *"OK" location 00490 00500 * GIVE A ZERO T0 BASIC: 00510 * SHOW RAM TEST OK 00520 0037 4F 00530 CLRA 0038 5F 00540 CLRB 0039 7E B4F4 00550 JMP $B4F4 *GIVABF 00560 00570 * ERROR: GIVE ERROR ADDRESS 00580 * BACK T0 BASIC 00590 003C B7 FFDE 00600 ERROR STA $FFDE *Se1ect Rom mode 003F 1F 10 00610 TFR X,D *Last Ram address to D 0041 7E B4F4 00620 JMP $B4F4 *and back to Basic's GIVABF 0000 00630 END 00000 TOTAL ERRORS