On 20 July 2017 at 13:11, Kirk Wolf <[email protected]> wrote:
> I'm actually writing this in Java (to 64-bit java "long" epoch seconds,
> from both STCK and STCKE inputs), but the sample IBM assembler code
> initially puzzled me.
I must admit I remain confused by both the sample code and its comments.
The the notion of "seconds per tod unit" is an odd one. This is a very
small number (about 0.000000000244140625), and it makes little sense to be
dividing anything by it. And certainly the constant EPOCST DC X'7A120000'
isn't it.
This is code I wrote in 1999 to do this. I must also admit I had not
considered that the Unix representation was signed, and so it doubtless
produces incorrect output for input dates between 2038 and 2042. But of
course any code does so by definition, so it's a matter of which failure
mode you prefer.
I like to think this is easy to read and understand. I think of the
algorithm this way:
Convert the TOD value into microseconds since Jan 1 1900. Why? Because it's
trivial and fast, it well fits the architected definition of the TOD clock
(bit 51 - 1 uS), and it guarantees that this intermediate value does not
have its high bit on.
Subtract the (constant) difference in microseconds between Jan 1 1900 and
Jan 1 1970. I hand calculated this by thinking (70 years * 365 days/year =
25550 days, + (70/4)-1 [-1 because 1900 was not a leap year] = 16 leap days
) = 25566 days ) * 1440 min/day *60 seconds/minute = 2208902400 seconds *
1000000 = 2208902400000000 microseconds. I hope I was right...
So here's my old code:
STCK WORK_DWORD
LM R14,R15,WORK_DWORD TOD CLOCK UNITS
SRDL R14,12 MICROSECONDS SINCE JAN 1, 1900
SL R15,=FL8'2208902400000000'+4 - RIGHT HALF
BC 11,*+6 BRANCH ON NO BORROW
BCTR R14,R0 -1 FOR BORROW
SL R14,=FL8'2208902400000000' - LEFT HALF
D R14,=F'1000000' SECONDS SINCE JAN 1, 1970
As others have pointed out, using the 32-bit registers and checking for
borrow would be strange for code written in 2017, but of course it still
works. Using a G register with the appropriate G Load, Shift, and Subtract
would eliminate 3 instructions.
> - good assembler programmer
> - XLC/C++ compiler
> - IBM sample code in a manual
> - bad assembler programmer
Heh - a colleague is writing his first ever, very simple, TSO command. The
"IBM sample code in a manual" has meant it's taken him about a week more
than it should've.
Tony H.
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