how to detect overflow with SUMC?

ManAtWork · 2021-01-09 14:27

The SUMC instruction is very useful if I want to add or subtract a value from a register based on the C flag. But how can I detect an over or underflow? I don't quite understand the stament "C = correct sign of (D +/- S)" in the docs.

What I like to do:
* if C is 0 then add S to D, set C if an overflow occurs (boundary $FFFFFFFF to $00000000 is crossed)
* if C is 1 then subtract S from D, set C if an underflow occurs (boundary $00000000 to $FFFFFFFF is crossed)

Is this possible with the SUMC instruction? Or do I have to split the operation into something like this?

  MODZ ... ' put C into Z
  if_z  SUB D,S wc
  if_nz ADD D,S wc

BTW, where are those constants defined that specify the actions for the MODC/Z instructions? Can't find them in the docs, again.

evanh · 2021-01-09 15:29

ManAtWork wrote: »

The SUMC instruction is very useful if I want to add or subtract a value from a register based on the C flag. But how can I detect an over or underflow? I don't quite understand the stament "C = correct sign of (D +/- S)" in the docs.

I've forgotten exactly. I remember the discussion over naming of that but the name, "correct sign", no longer makes it clear for me either. TonyB put a lot of effort into ensuring the carry flag functioned better in the prop2.

EDIT: Here's the original conversation - http://forums.parallax.com/discussion/167295/c-after-cmps-cmpsx-adds-addsx-subs-subsx/p1

BTW, where are those constants defined that specify the actions for the MODC/Z instructions? Can't find them in the docs, again.

The list is in instructions.txt in PNut zip, and now also added to end of main hardware google doc.

Wuerfel_21 · 2021-01-09 16:05

ManAtWork wrote: »

What I like to do:
* if C is 0 then add S to D, set C if an overflow occurs (boundary $FFFFFFFF to $00000000 is crossed)
* if C is 1 then subtract S from D, set C if an underflow occurs (boundary $00000000 to $FFFFFFFF is crossed)

This should be what you're looking for (untested):

sumc d,s wc
testb d,#31 xorc

evanh · 2021-01-09 16:17

Wuerfel_21 wrote: »
This should be what you're looking for (untested):
sumc d,s wc
testb d,#31 xorc

Looks right as per http://forums.parallax.com/discussion/comment/1421463/#Comment_1421463

ManAtWork · 2021-01-09 21:41

Wuerfel_21 wrote: »
This should be what you're looking for (untested):
sumc d,s wc
testb d,#31 xorc

I've just tested it. The idea is good but the results are a bit different:
* if C is 0 then add S to D, set C if a signed overflow occurs (boundary $7FFFFFFF to $80000000 is crossed)
* if C is 1 then subtract S from D, set C if a signed underflow occurs (boundary $80000000 to $7FFFFFFF is crossed)

However, that is not a problem for me. I can just add an offset to D. The exact number where the C bit is set doesn't really matter. It's only important that it happens once per "revolution" and at the same point when turning backwards.

evanh · 2021-01-09 23:36

Yeah, SUMxx instructions will be signed. You can tell because in the instruction sheet it says C = correct sign, where as for unsigned instructions it says C = carry.

TonyB_ · 2021-01-09 23:49

Just to add, "correct sign" means that even if there is an overflow and therefore the result is nonsense, C is always the the correct sign for any signed addition/subtraction, thus one can do a compare and not worry about a possible overflow. I came up with the term "correct sign" as it starts with the letter C.

Wuerfel_21 · 2021-01-10 00:00

An alternative that I think would actually trigger at the zero-crossing, but requires the Z flag (still untested):

testb d,#31 wz
sumc d,s wc
' now use if_diff/if_same conditions instead of if_c/if_nc

TonyB_ · 2021-01-10 10:36

Don't forget about TJV !

cgracey · 2021-01-10 10:54

TonyB_ wrote: »

Don't forget about TJV !

Yes, TJV is what they're looking for.

ManAtWork · 2021-01-10 11:39

AFAICS, TJV does test for the exact same condition as Wuerfels proposal.

TJV Test D and jump to S** if D overflowed (D[31] != C, C = 'correct sign' from last addition/subtraction).

I mean, "D[31]!=C" is the same as "testb d,#31 xorc".

Thanks for the solution

TonyB_ · 2021-01-10 12:31

TJV is useful for more than testing overflow.

evanh · 2021-01-11 07:41

I've dug up the old test code for this. It was very early stuff so needed some work to interface with my current setup. Here's a fresh set results with some extras added:

00000004(reg1) + 00000005(reg2) = 00000009
00000004(reg1) - 00000005(reg2) = ffffffff
Collected flags = 0000028a
ADD  reg1,reg2:  C = 0,  Z = 0
SUB  reg1,reg2:  C = 1,  Z = 0
CMP  reg1,reg2:  C = 1,  Z = 0
ADDS reg1,reg2:  C = 0,  Z = 0
SUBS reg1,reg2:  C = 1,  Z = 0
CMPS reg1,reg2:  C = 1,  Z = 0

00000005(reg1) + 00000004(reg2) = 00000009
00000005(reg1) - 00000004(reg2) = 00000001
Collected flags = 00000000
ADD  reg1,reg2:  C = 0,  Z = 0
SUB  reg1,reg2:  C = 0,  Z = 0
CMP  reg1,reg2:  C = 0,  Z = 0
ADDS reg1,reg2:  C = 0,  Z = 0
SUBS reg1,reg2:  C = 0,  Z = 0
CMPS reg1,reg2:  C = 0,  Z = 0

80000000(reg1) + 80000000(reg2) = 00000000
80000000(reg1) - 80000000(reg2) = 00000000
Collected flags = 00000d75
ADD  reg1,reg2:  C = 1,  Z = 1
SUB  reg1,reg2:  C = 0,  Z = 1
CMP  reg1,reg2:  C = 0,  Z = 1
ADDS reg1,reg2:  C = 1,  Z = 1
SUBS reg1,reg2:  C = 0,  Z = 1
CMPS reg1,reg2:  C = 0,  Z = 1

7fffffff(reg1) + 7fffffff(reg2) = fffffffe
7fffffff(reg1) - 7fffffff(reg2) = 00000000
Collected flags = 00000145
ADD  reg1,reg2:  C = 0,  Z = 0
SUB  reg1,reg2:  C = 0,  Z = 1
CMP  reg1,reg2:  C = 0,  Z = 1
ADDS reg1,reg2:  C = 0,  Z = 0
SUBS reg1,reg2:  C = 0,  Z = 1
CMPS reg1,reg2:  C = 0,  Z = 1

7fffffff(reg1) + 80000000(reg2) = ffffffff
7fffffff(reg1) - 80000000(reg2) = ffffffff
Collected flags = 000002a0
ADD  reg1,reg2:  C = 0,  Z = 0
SUB  reg1,reg2:  C = 1,  Z = 0
CMP  reg1,reg2:  C = 1,  Z = 0
ADDS reg1,reg2:  C = 1,  Z = 0
SUBS reg1,reg2:  C = 0,  Z = 0
CMPS reg1,reg2:  C = 0,  Z = 0

80000000(reg1) + 7fffffff(reg2) = ffffffff
80000000(reg1) - 7fffffff(reg2) = 00000001
Collected flags = 0000002a
ADD  reg1,reg2:  C = 0,  Z = 0
SUB  reg1,reg2:  C = 0,  Z = 0
CMP  reg1,reg2:  C = 0,  Z = 0
ADDS reg1,reg2:  C = 1,  Z = 0
SUBS reg1,reg2:  C = 1,  Z = 0
CMPS reg1,reg2:  C = 1,  Z = 0

80000000(reg1) + 80000001(reg2) = 00000001
80000000(reg1) - 80000001(reg2) = ffffffff
Collected flags = 00000aaa
ADD  reg1,reg2:  C = 1,  Z = 0
SUB  reg1,reg2:  C = 1,  Z = 0
CMP  reg1,reg2:  C = 1,  Z = 0
ADDS reg1,reg2:  C = 1,  Z = 0
SUBS reg1,reg2:  C = 1,  Z = 0
CMPS reg1,reg2:  C = 1,  Z = 0

80000001(reg1) + 80000000(reg2) = 00000001
80000001(reg1) - 80000000(reg2) = 00000001
Collected flags = 00000820
ADD  reg1,reg2:  C = 1,  Z = 0
SUB  reg1,reg2:  C = 0,  Z = 0
CMP  reg1,reg2:  C = 0,  Z = 0
ADDS reg1,reg2:  C = 1,  Z = 0
SUBS reg1,reg2:  C = 0,  Z = 0
CMPS reg1,reg2:  C = 0,  Z = 0

7fffffff(reg1) + ffffffff(reg2) = 7ffffffe
7fffffff(reg1) - ffffffff(reg2) = 80000000
Collected flags = 00000a80
ADD  reg1,reg2:  C = 1,  Z = 0
SUB  reg1,reg2:  C = 1,  Z = 0
CMP  reg1,reg2:  C = 1,  Z = 0
ADDS reg1,reg2:  C = 0,  Z = 0
SUBS reg1,reg2:  C = 0,  Z = 0
CMPS reg1,reg2:  C = 0,  Z = 0

ffffffff(reg1) + 7fffffff(reg2) = 7ffffffe
ffffffff(reg1) - 7fffffff(reg2) = 80000000
Collected flags = 0000080a
ADD  reg1,reg2:  C = 1,  Z = 0
SUB  reg1,reg2:  C = 0,  Z = 0
CMP  reg1,reg2:  C = 0,  Z = 0
ADDS reg1,reg2:  C = 0,  Z = 0
SUBS reg1,reg2:  C = 1,  Z = 0
CMPS reg1,reg2:  C = 1,  Z = 0

how to detect overflow with SUMC?

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