RAM test: was Re: PET startup sequence??

On Aug 14 2004, 11:11, Vintage Computer Festival wrote: can OK, it's below. First I ought to explain a couple of things about the code and the system it was written for. This was for a very basic Z80 system consisting of some EPROM, RAM, timers, and minimal I/O, which included a 2-line x 16-char LCD display. It was used for second-year project work. The way the LCD display was interfaced precluded the use of normal hanshaking or reading, so the routines that write to it incorporate a delay long enough to ensure that the display has completed the update before another write can occur. Secondly, the whole system was designed to report any error (and refuse to continue if it was serious), and to do this, the startup routines perform a number of tests, each of which uses no part of the hardware that's not already been passed "good". Well, that's the ideal; it's not so simple in practice. It begins with a ROM CRC check (and it's a proper CRC, not a simple checksum, so it will catch all single-bit errors, most 2-bit errors, and many multiple-bit errors). It should really do a CPU test first, but that would need the ROM to hold the test routines. The justification here is that it uses nothing except the processor and the ROM itself, and the assumption is that any serious fault in the processor will cause that to fail. If so, it's easy to take out the ROM, CRC check it on an EPROM programmer, and tell whether the ROM really is at fault -- if not, it must be the CPU. The chances of a CPU failing are quite small; the chances that a failing CPU will still execute instructions are a fraction of that; the chances that a CPU with faulty instructions will still execute the CRC test are a fraction of that; the chances that a faulty CPU that still passes the CRC has faults in common instructions used elsewhere are ... etc. So at the point that the RAM test executes, we "know" the CPU is OK and we know the ROM is OK, but we don't want to use the RAM to hold data during the RAM test. So no variables other than in registers, and *no stack* hence no subroutines. Hence the rather odd way of executing pseudo-subroutines to display things (on the LCD which we assume is working; it's just been "exhaustively tested" by the visual test of seeing a "ROM CRC ERROR" or "ROM CRC PASSED" message, followed by "RAM test" on it :-)). Lastly, this test does everything one bit at a time, not one byte at a time. That's because the system used byte-wide RAM, in which in theory any bit could interact with any other. You could safely parallelise this to do a byte at a time if you used a separate RAM chip for each bit (because you could assume they wouldn't interact except through a faulty data bus or power glitches) and that would speed it up more than 8 times. Here's the code: # Filename: RAMtest.s # # Author: Pete Turnbull # Address: Department of Computer Science, University of York, # Heslington, YORK YO10 5DD. # Email: pnt1(a)york.ac.uk # # Created: 26-Nov-1995 # Last update: 18-Nov-1996 # Description: An efficient RAM test # Version: 0.4 now uses 20-bit error-counter # 0.3 modified to work as standalone test for CTS 19-Oct-1996 # 0.2 code converted from ZASM to as80 for MCP, March 1996 # 0.1 original module for inclusion in CTS 1995, coded in ZASM # # ************************************************************************** # * # * An implementation of Suk and Reddy's Test B RAM testing procedure. # * Implemented from information in an article "Functional Testing of # * Semiconductor Random Access Memories", M.S.Abadir and H.K.Reghbati, # * Computing Surveys, Vol.15 No.3 September 1983, ACM. # * The original NTA article is "Efficient Algorithms for testing # * Semiconductor Random Access Memories", Nair, Thatte and Abraham, IEEE # * Transactions on Computing, Vol.C-27, No.6, June 1978. # * A similar fault model was used by D.S.Suk and S.M.Reddy, to develop even # * better tests, including their Test B, described in "A march test for # * functional faults in semiconductor random access memories", IEEE # * Transactions on Computing, Vol.C-30, No.12, Dec 1981. # * This is an O(n) procedure, much more efficient for large memories than # * GALPAT. It also finds coupling faults invisible to GALPAT tests. # * # * GALPAT complexity 4n^2 + 2n, finds all stuck-at, and some coupling faults # * GALPAT-II 4n^2 + 4n, finds all stuck-at, and all coupling faults # * T&A 8n.lg(n), finds all stuck-at, and all coupling faults # * NTA 30n, finds all stuck-at, and all coupling faults # * S&R-B 16n, finds all stuck-at, and all coupling faults # * assuming no decoder multiple-access faults, # * and all decoder faults if no coupling faults. # * # * The proviso about coupling/decoder faults really just means that the two # * types of fault are seen as equivalent (indistinguishable) by the test, # * not that they are not detected. # * # * There are even faster tests than S&R-B but they don't necessarily find # * all errors (some errors may mask others). # * With our Z80 running at 3.6864MHz, this test takes about 6 seconds. # * From the formulae above, an "equivalent" GALPAT would take over an hour. # * # * Byte operations would be acceptable for 1-bit wide memories, merely # * being an application of parallel testing. However, our RAM is 8-bit, so # * we use multiple operations to access each bit in a byte. This is not # * quite according to the test specification, as the lower bits in each # * byte are read/written up to 21 more times than they should be. # * However, if the RAM is OK, correct values are written every time, and # * consideration of the fault model shows that this will only affect soft # * errors, which, by definition, are likely to be corrected by other # * operations in the test procedure and would not be detected by most test # * algorithms. # * If we just used byte operations, we might miss some coupling faults. # * # * Errors are reported on the LCD, using in-line code (no subroutine stack), # * in the form "aaaa:xx (yy) -pp" where "aaaa" is the address with the error, # * "xx" is the found data, "yy" is the expected data, and "pp" is the section # * number (1A, 1B, 1C, 2, 3 or 4). DE holds an error count, supplemented by # * H to use 20 bits so that multiples of 8*8192 errors (eg every bit in every # * byte of 8K) can be recorded. # * # * The test always runs to completion, but then displays the error count and # * HALTs if it's non-zero. If it is zero, this version prints the message # * "RAM OK " and halts. # * # ************************************************************************** # # # ***** Memory addresses ***** # rom = 0 # start of 8KB EPROM area romtop = 0x1FFF # last byte in ROM ram = 0x4000 # start of 8KB RAM area ramsize = 8192 # size of RAM ramtop = ram+ramsize-1 # last address in RAM # # # ***** I/O addresses ***** # LCDins = 0x80 # LCD base address, instruction register LCDdat = LCDins+1 # LCD data register # # # ***** assorted constants ***** # LCinit = 0x3C # LCD "Funct.Set": 8 bit data, 2 lines LCDoff = 0x08 # set off: OR this with D/C/B for ON Don = 4 # sets Display ON LCDon = LCDoff + Don # set on: OR this with C/B for cursor Con = 2 # sets Cursor ON Bon = 1 # sets Blink ON (else cursor is solid) LCDclr = 0x01 # clears LCD display/RAM, sets address 0 LCDhom = 0x02 # moves cursor to position 1, line 1 setCG = 0x40 # sets "write to char gen RAM". OR with addr. setDD = 0x80 # sets "write to data display". OR with addr. line2 = 0x40 # address of second line setL2 = setDD + line2 # sets "write" to write to line 2 # dly120 = 32 # loop constant for just under 120 microsecs space = ' ' # ASCII space character # # # ************************************************************************** # * # * Here is the actual code... # * Note there's no point in pushing registers or calling subroutines, # * as we're scribbling all over all the RAM including the stack area. # * Before we start, reset LCD and tell the world what we're going to do. # * # ************************************************************************** # .org rom init: ld a, LCinit # tell it about data format etc out (LCDins), a ld b, dly120 waitI0: djnz waitI0 # let the LCD sort itself out ld a, LCDclr # clear display and RAM, home cursor out (LCDins), a ld bc, 4900/7 # that takes about 4.9ms waitLC: dec bc # loop takes 26 T-cycles, about 7 microsecs ld a, b or c jr nz, waitLC # wait for init to complete ld a, LCDon + Con # turn on the display out (LCDins), a ld b, dly120 waitI1: djnz waitI1 # let the LCD get it done ld hl, SRBmsg jp mssgHL # and return from there to following code SRBmsg: "RAM test " # this message fills the first line .byte 0 # terminator # # # ***** Step 0. Initialise, and set all RAM to 0's (we hope). # 8Kbytes-worth of LDIR takes 21 T-cycles x 8192, about 47 milliseconds # SRB0: ld de, 0 # assume we're going to pass - no errors ld h, d # (error counter is 24-bit) exx # save that thought! ld hl, ram ld (hl), 0 # set first location to zero ld bc, ramsize - 1 # make 1FFF copies... ld de, ram + 1 # ...starting here ldir # block copy, fastest way to set all the rest # # ***** Step 1. 3 pairs of read/write operations for each *bit* (not byte). # This is a marching pattern, but tests that each bit can be 1 or 0. # For sake of speed, loops are done with JP, faster than JR if jump # is made. However, JR is faster if the jump is not taken, so is used for # the jump-on-failure -- which hopefully is rarely taken! # "nxtbit" loop takes 110 T-cycles, about 30 microseconds. # "nxtbyt" loop takes 8 nxtbits + 60 T-cycles = 940 T-cycles, about 255us # 8Kbytes-worth takes 255us x 8192 = 2.09 seconds. # ld bc, ramtop + 1 # where to stop ld d, 1 # mask for current bit in current byte ld hl, ram-ramtop-1 nxtbyt: add hl, bc # start position ld e, 0 # what the whole of the current byte should be nxtbit: ld a, (hl) # ** Read: Ci(=0) and d # select current bit jr nz, fail1a # if not still 0, it's duff OK1a: ld a, e # what it should be, so far or d # ** Write: Ci <-- 1 ld (hl), a # put it away ld a, (hl) # ** Read: Ci(=1) and d jr z, fail1b OK1b: ld a, e # current bit is still zero in E ld (hl), a # ** Write: Ci <-- 0 ld a, (hl) # ** Read: Ci(=0) and d # select current bit again jr nz, fail1c # if not still 0, it's duff OK1c: ld a, e # what it should be, so far or d # ** Write: Ci <-- 1 ld e, a # update the copy ld (hl), a # put it to bed rlc d # next bit - NB 8-bit rotate jp nc, nxtbit inc hl ccf # it was set by the last RLC D sbc hl, bc # see if we've run out of RAM jp nz, nxtbyt # # ***** Step 2. Check each bit Ci(=1), then toggle it twice. # "nxtbi2" loop takes 62 T-cycles, about 17 microseconds. # "nxtby2" loop takes 8 nxtbits + 46 T-cycles = 542 T-cycles, about 147us # 8Kbytes-worth takes 147us x 8192 = 1.2 seconds. # ld hl, ram-ramtop-1 nxtby2: add hl, bc # E is still FF, this section doesn't change it nxtbi2: ld a, (hl) # ** Read: Ci(=1) and d jr z, fail2 OK2: ld a, e # what it should be (0FFH) xor d # make current bit zero ld (hl), a # ** Write Ci <-- 0 ld a, e # restore the '1' ld (hl), a # ** Write Ci <-- 1 rlc d jp nc, nxtbi2 inc hl ccf # it was set sbc hl, bc # see if we've run out of RAM jp nz, nxtby2 # # ***** Step 3. Check each bit Ci(=1), then toggle it three times. # "nxtbi3" loop takes 77 T-cycles, about 21 microseconds. # "nxtby3" loop takes 8 nxtbits + 46 T-cycles = 662 T-cycles, about 180us # 8Kbytes-worth takes 180us x 8192 = 1.5 seconds. # ld hl, ram-ramtop-1 nxtby3: add hl, bc ld e, 0xFF # it gets changed to 0 as we go round nxtbi3: ld a, (hl) # ** Read: Ci(=1) and d jr z, fail3 OK3: ld a, e # what the byte should be xor d # make current bit zero ld (hl), a # ** Write Ci <-- 0 ld a, e # restore the '1' ld (hl), a # ** Write Ci <-- 1 xor d # make current bit zero ld e, a # copy it ld (hl), a # ** Write Ci <-- 0 rlc d jp nc, nxtbi3 inc hl ccf # it was set sbc hl, bc # see if we've run out of RAM jp nz, nxtby3 # # ***** Step 4. Check each bit Ci(=0), then toggle it twice. # "nxtbi4" loop takes 62 T-cycles, about 17 microseconds. # "nxtby4" loop takes 8 nxtbits + 46 T-cycles = 542 T-cycles, about 147us # 8Kbytes-worth takes 147us x 8192 = 1.2 seconds. # ld hl, ram-ramtop-1 nxtby4: add hl, bc # E is now zero, after finishing Step 3 nxtbi4: ld a, (hl) # ** Read: Ci(=0) and d jr nz, fail4 OK4: ld a, e # what it should be (00H) xor d # make current bit a '1' ld (hl), a # ** Write Ci <-- 1 ld a, e # restore the '0' ld (hl), a # ** Write Ci <-- 0 rlc d jp nc, nxtbi4 inc hl ccf # it was set sbc hl, bc # see if we've run out of RAM jp nz, nxtby4 jp SRBend # testing completed # # # ************************************************************************** # * # * The following code uses IX to store the return address. It uses BC, # * but restores BC = ramtop + 1 on exit. It prints a message to give # * the failed address, data as read, expected data, and section where # * the error was detected. # * # ************************************************************************** # fail1a: ld ix, OK1a # here if Read: Ci(=0) failed in Step 1a ld a, 0x1A # section code jr fail fail1b: ld ix, OK1b # here if Read: Ci(=1) failed in Step 1b ld a, 0x1B jr fail fail1c: ld ix, OK1c # here if Read: Ci(=0) failed in Step 1c ld a, 0x1C jr fail fail2: ld ix, OK2 # here if Read: Ci(=0) failed in Step 2 ld a, 2 jr fail fail3: ld ix, OK3 # here if Read: Ci(=1) failed in Step 3 ld a, 3 jr fail fail4: ld ix, OK4 # here if Read: Ci(=0) failed in Step 4 ld a, 4 # # ***** This is the part that actually does the printing # fail: ex af, af # save section code for the moment ld a, setL2 out (LCDins), a # address LCD second line ld b, dly120 fwait1: djnz fwait1 # give it time to do its stuff ld c, h # upper byte of failing address ld iy, f.L jr f.hex # print it f.L: ld c, l # lower byte of failing address ld iy, f.coln jr f.hex f.coln: ld a, ':' # print colon out (LCDdat), a ld b, dly120 fwait2: djnz fwait2 ld c, (hl) # get the duff data ld iy, f.spc jr f.hex f.spc: ld a, space # print space out (LCDdat), a ld b, dly120 fwait3: djnz fwait3 ld a, '(' # print left bracket out (LCDdat), a ld b, dly120 fwait4: djnz fwait4 ld c, e # expected data ld iy, f.brk jr f.hex f.brk: ld a, ')' # print right bracket out (LCDdat), a ld b, dly120 fwait5: djnz fwait5 ld a, space # print space out (LCDdat), a ld b, dly120 fwait6: djnz fwait6 ld a, '-' # print a dash out (LCDdat), a ld b, dly120 fwait7: djnz fwait7 ex af, af # where we saved the section number ld c, a ld iy, f.fin jr f.hex f.fin: exx # remember H,DE' had zero to signify "pass" inc e # so update error count jr nz, f.exx inc d # update middle byte too, if necessary jr nz, f.exx inc h # update top byte if necessary f.exx: exx ld bc, ramtop + 1 # caller expects this to be here jp (ix) # back to "caller" (or wherever!) # # # ************************************************************************** # * # * This pseudo-subroutine is "called" to print a pair of hex digits. # * # * Entry: value in C, return address in IY. # * Exit: A and B mangled. # * # ************************************************************************** # f.hex: ld a, c # get value to print rrca # move upper nibble to lower rrca rrca rrca and 0x0F # remove any garbage cp 0x0A # do we need a letter? jr c, AOK.1 add a, 'A' - 0x3A # yes, adjust it AOK.1: add a, '0' # make it ASCII out (LCDdat), a # print it ld b, dly120 Await1: djnz Await1 # give LCD time to do his stuff f.hex1: ld a, c # get lower nibble this time and 0x0F cp 0x0A jr c, AOK.2 add a, 'A' - 0x3A AOK.2: add a, '0' out (LCDdat), a ld b, dly120 Await2: djnz Await2 jp (iy) # return to wherever caller specified # # ************************************************************************** # * # * This pseudo-subroutine prints a message (if the as-yet-untested LCD is # * working) -- for use by the test routines that have no stack. # * # * Place the required message immediately after the call; this code returns # * to the address immediately beyond the message. # * # * Entry: HL points to the message, zero-terminated # * Exit: A, B mangled, HL updated # * # ************************************************************************** # mssgHL: ld a, (hl) # get the character or a # is it the terminator? inc hl # doesn't affect the flags... jr nz, do.mhl # ...so this depends on the character jp (hl) # to the code following the message do.mhl: out (LCDdat), a # send character to LCD ld b, dly120 waitm: djnz waitm # wait for our slow LCD jr mssgHL # repeat until cooked # # # ************************************************************************** # * # * Arrive here on completion of RAM testing # * # ************************************************************************** # SRBend: ld a, setDD + 4 # to replace "test" on LCD with count out (LCDins), a ld b, dly120 fwait8: djnz fwait8 # give LCD time to do his stuff exx # alternate reg set had DE = error count ld a, h # ...see if it passed or d or e jp nz, SRBerr # skip if errors ld hl, SRB.OK # tell the user all is well jp mssgHL SRB.OK: "OK " .byte 0 # marks end of message halt # we're all done here # # ***** do this if RAM failed test # SRBerr: ld c, h # error count, MSByte ld iy, SRBE.M jr f.hex1 # only show 5 digits, to fit 16-char display SRBE.M: ld c, d # error count, middle byte ld iy, SRBE.L jr f.hex SRBE.L: ld c, e # error count, LSByte ld iy, SRB.em jr f.hex SRB.em: ld hl, errors # message " errors" jp mssgHL errors: " errors" # with 20-bit count, just fits 16-char display .byte 0 # marks end of message SRBhlt: halt # unwise to carry on with duff RAM -- Pete Peter Turnbull Network Manager University of York