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Author Topic: divison by zero?  (Read 5423 times)
masterj
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« on: January 12, 2013, 10:51:44 AM »

Hi, guys!
Just a small question here: how does ME7 (and ME7.5) ecu react to division by zero? For example there's certain situation when I have to set some variable to zero, but only way I can control this variable with is by division:

uncontrollable_var / MAP = var_needs_to_be_zero

So I was thinking, maybe it is possible to set MAP = 0 to force div by zero... maybe then ecu will set var_needs_to_be_zero = 0? Or it can send something like NaN and result in shutdown??
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rajivc666
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« Reply #1 on: January 12, 2013, 01:17:30 PM »

This is how the processor reacts, interesting  never been in a similar  situation, can you give the details.

Condition
Flags
E Z V C N
0 * S 0 *
E Always cleared.
Z Set if result equals zero. Cleared otherwise.
V Set if an arithmetic overflow occurred, i.e. if the divisor (op1)
was zero (the result in MDH and MDL is not valid in this case).
Cleared otherwise.

C Always cleared.
N Set if the most significant bit of the result is set. Cleared
otherwise.
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masterj
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« Reply #2 on: January 12, 2013, 01:37:35 PM »

This is how the processor reacts, interesting  never been in a similar  situation, can you give the details.

Condition
Flags
E Z V C N
0 * S 0 *
E Always cleared.
Z Set if result equals zero. Cleared otherwise.
V Set if an arithmetic overflow occurred, i.e. if the divisor (op1)
was zero (the result in MDH and MDL is not valid in this case).
Cleared otherwise.

C Always cleared.
N Set if the most significant bit of the result is set. Cleared
otherwise.

Well, I'm trying to achieve phlsnh = 0. One way is to set UBHS = 0, but that would also kill precat sensor heater voltage thus disabling both sensors completely Smiley Just found out this today, never thought that one map can be shared between two functions, but looks like UBHS is. So, the other way to set phlsnh = 0 is to somehow circumvent it in the UB_KORR3 subfunction... OR if possible to change ASM code to use constant zero instead of UBHS map... Hm... that might be interesting solution...

Code:
seg003:54D1A sub_854D1A:
seg003:54D1A                 mov     [-r0], r9
seg003:54D1C                 mov     [-r0], r8
seg003:54D1E                 mov     [-r0], r7
seg003:54D20                 mov     [-r0], r6
seg003:54D22                 sub     r0, #4
seg003:54D24                 jnb     word_FD9A.1, loc_854D3A
seg003:54D28                 movb    rl4, ub         ; ub
seg003:54D2C                 extp    #206h, #1
seg003:54D30                 cmpb    rl4, UBHS       ; UBHS
seg003:54D34                 jmpr    cc_UGT, loc_854D3A
seg003:54D36                 bset    USR0
seg003:54D38                 jmpr    cc_UC, loc_854D3C

Could I just do something like this:
Code:
seg003:54D30                 cmpb    rl4, #0       ; set to #0
« Last Edit: January 12, 2013, 01:46:20 PM by masterj » Logged

rajivc666
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« Reply #3 on: January 12, 2013, 02:02:47 PM »

If you think it will work then change the hex at 0x54D22 to
4980                  cmpb rl4,#0
CC00                  nop
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masterj
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« Reply #4 on: January 12, 2013, 02:10:18 PM »

If you think it will work then change the hex at 0x54D22 to
4980                  cmpb rl4,#0
CC00                  nop

Thank you! I couldn't figure out how to set constant in asm->hex Smiley

BTW: cmpb is same: 41, 43, 47, 49, no?

Also #0 -> 80? How would constant #3 would look in hex?

If I wanted say to create UBHS_2 @ 8A6812 and point the said earlier line to this address instead of original UBHS, how would I have to change this code?
« Last Edit: January 12, 2013, 02:14:45 PM by masterj » Logged

masterj
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« Reply #5 on: January 18, 2013, 01:28:43 PM »

One more thing that is very interesting to me:
For example in me7logger ecu file im looking at the:
Code:
B_hshe          , {}                                , 0x00FD3C,  2,  0x0008, {}        , 0, 0,            1,      0, {Bedingung Endstufe Sondenheizung hinter Kat angesteuert}

So, B_hshe is @ FD3C (IRAM?)? Also how to decode bitmask? Is it FD3C.3?

BTW: How do you rename variable bit in IDA??? Like word_FD3C.1 I want rename to var1, word_FD3C.2 to var2 ...
« Last Edit: January 18, 2013, 02:16:23 PM by masterj » Logged

masterj
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« Reply #6 on: January 19, 2013, 02:36:49 PM »

If you think it will work then change the hex at 0x54D22 to
4980                  cmpb rl4,#0
CC00                  nop

Works as expected. Thank you rajivc666 Smiley I can confirm now that this patch disables post cat o2 heater flicker ON/OFF in vcds - now it stays off all the time Wink

P.s> Address was 54D30 Wink
« Last Edit: January 19, 2013, 02:40:19 PM by masterj » Logged

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