PDLAMCH(l)

✨ NAME ✨

okay so bestie, meet PDLAMCH 💅 — the LAPACK auxiliary routine whose whole entire job is to determine double precision machine parameters. that's it. that's the gig. she's not doing side quests, she's not doing your taxes, she's just telling you what your machine's double precision vibes are. 👑

🌸 SYNOPSIS 🌸

DOUBLE PRECISION FUNCTION PDLAMCH( ICTXT, CMACH )

    CHARACTER    CMACH

    INTEGER      ICTXT

🦋 PURPOSE 🦋

PDLAMCH determines double precision machine parameters. literally her only character trait and honestly? iconic. no cap. ✨

🎀 ARGUMENTS 🎀

• ICTXT (global input) INTEGER — the BLACS context handle, aka the specific little universe where the computation takes place. think of it as the group chat this whole thing is happening in. 🫶
• CMACH (global input) CHARACTER*1 — this one specifies the value to be returned by PDLAMCH, and you get to pick your fighter:
  • = 'E' or 'e', PDLAMCH := eps
  • = 'S' or 's , PDLAMCH := sfmin
  • = 'B' or 'b', PDLAMCH := base
  • = 'P' or 'p', PDLAMCH := eps*base
  • = 'N' or 'n', PDLAMCH := t
  • = 'R' or 'r', PDLAMCH := rnd
  • = 'M' or 'm', PDLAMCH := emin
  • = 'U' or 'u', PDLAMCH := rmin
  • = 'L' or 'l', PDLAMCH := emax
  • = 'O' or 'o', PDLAMCH := rmax

where

• eps — relative machine precision
• sfmin — safe minimum, such that 1/sfmin does not overflow
• base — base of the machine
• prec — eps*base
• t — number of (base) digits in the mantissa
• rnd — 1.0 when rounding occurs in addition, 0.0 otherwise
• emin — minimum exponent before (gradual) underflow
• rmin — underflow threshold - base**(emin-1)
• emax — largest exponent before overflow
• rmax — overflow threshold - (base**emax)*(1-eps)

ngl bestie, those are the definitions and you're gonna want them memorized, because CMACH is a single character and one wrong letter is a whole different number. 🙈

and that's the whole page, fr fr. 💜 this one is from LAPACK version 1.5, dated 12 May 1997 — yes she's vintage, yes she still ate, and she's serving PDLAMCH(l) realness to this day. 🎀✨💖