HP Forums

Full Version: MCODE question - C=-C-1 MS sets carry in MLDL but not in SDK41
You're currently viewing a stripped down version of our content. View the full version with proper formatting.

To the MCODE gurus

There seems to be an interesting discrepancy between the behavior of MCODE in the 41 (currently using a HEPAX) and in SDK41. Given that the SDK41 uses the original roms, this surprised me.

Even more surprised am I that the instruction C=-C-1 MS can create a carry with a very benign positive number in C (MS=0). The code below runs as shown in SDK41 and produces for 100 in x-reg the correct value of 71. However when run within an MLDL it produces -29 as the ?NCXQ in line x11F and x124 are not run. If I insert a NOP infront of them the code runs in the MLDL as well, producing the correct result.

Please see the code snippet below (the code does not do anything useful, it’s a preamble to a Prime-Twin generator, my usual way of learning a new language). Any thoughts with regards to

  • How C=-C-1 for a benign positive number can set the carry
  • Difference between SDK41 and MLDL
would be much appreciated.

Thanks so much !



;calculate startvalue for search for prim-twins
;given a number in X, startvalue = 30*Int((X+19)/30) – 19
;implemented as (X+19) – (X+19)Mod30 -19
Add HexCode Command Comment
x107 2A0 SetDec
x108 1A0 A=B=C=0
x109 35C R=12 ;load 19 into C
x10A 050 LD@R 1
x10B 250 LD@R 9
x10C 226 C=C+1 S&X
x10D 070 N=C All ;save for later use
x10E 10E A=C All ;prepare regs for AD2_10
x10F 0F8 Read 3(x)
x110 01D060 ?NCXQ [AD2_10] ;this one works fine
x112 10E A=C All ;A=X+19
x113 158 M=C All ;save in M for later
x114 04E C=0 All ;Load 30 into C to prep for [MOD10]
x115 35C R=12
x116 0D0 LD@R 3
x117 226 C=C+1 S&X
x118 171064 ?NCXQ [MOD10] ;A Mod C
x11A 0EE B<>C ;prep for subtraction via AD2_10
x11B 198 C=M All ;recall X+19
x11C 10E A=C All
x11D 0CE C=B All ;(X+19) Mod 30
x11E 2BE C=-C-1 MS ;This should simply make it negative. But it sets Carry!!
x11F 01D060 ?NCXQ [AD2_10] ;this only works if I have a NOP in front to clear carry when run in MLDL
;it calcs (X+19) – (X+19)Mod30
x121 10E A=C All ;prep to subtract 19
x122 0B0 C=N All ;recall 19 from N
x123 2BE C=-C-1 ;same problem it sets carry when run in MLDL
x124 01D060 ?NCXQ [AD2_10]
x126 0E8 WRIT 3(x) ;save result to x
x127 3E0 RTN

I don't know anything about SDK41, but you should expect to always get the carry flag set as the result of c=-c-1. For subtract instructions, the carry flag indicates a "borrow" out of the leftmost digit. What you're really doing is subtracting from zero, and that will always have a borrow. If you were working on a two-digit field of C that contained 23, after c=-c-1 you should have 76 with a carry.

In the hardware, subtraction is really implemented by addition of a nine's complement (assuming decimal mode), with both the carry-in and carry-out inverted. For the normal add and subtract instructions, the carry-in is zero, but for 0=-c-1 the carry-in is one (before being inverted).

that makes sense! Thanks Eric for helping me out!

I got confused as in Ken's book the instruction C=-c-1 is explained as 'doing the nine's complement' (p26) and I picked up the 'trick' in one of the books but I must have overlooked the NOP before the jump instruction there.. Anyway, thanks a lot again!