B side: Organizing a program

B B side · Fortran · The language that speaks maths

How do you divide a long text into parts with names?

Flip to the A side (Assembly): Organizing a program →

16 Fortran words in this segment

11 Assembly words for the same job

What Fortran says well here

• Modules, subroutines, functions and INTENT keep large programs clear.
• The compiler makes sure that each call matches the procedure.
• Each table that the B side makes gets a wrong twin: the same program with one wrong word or value. The build requires the check to fail on the twin, so the check itself is tested.

What is hard to say

• You write more words before the first result.

One sentence in Fortran

module physics
  implicit none
contains
  pure function next_speed(v, a, dt)
    real, intent(in) :: v, a, dt
    real :: next_speed
    next_speed = v + a*dt
  end function
end module

It says: A module named physics with one function that calculates the next speed.

The words, by part of speech

Verbs (1) · A statement that does something, for example PRINT, CALL or ALLOCATE.

Word	Meaning	Example
CALL	Run a subroutine.	call draw(ship)

Adjectives (5) · An attribute, for example ALLOCATABLE or PARAMETER.

Word	Meaning	Example
INTENT(IN)	This argument comes in, and the procedure must not change it.	real, intent(in) :: dt
INTENT(OUT)	The procedure gives this argument its value.	real, intent(out) :: v2
PURE	This procedure changes nothing outside itself.	pure function f(x)
ELEMENTAL	Write the procedure for one value. Fortran then applies it to each value of an array.	elemental function f(x)
PRIVATE / PUBLIC	Hide a name inside its module, or show it to other code.	private :: helper

Grammar marks (2) · Maths signs (+ - * / **), the = sign, and (:) for part of an array.

Word	Meaning	Example
!	Starts a comment. The compiler ignores the rest of the line.	! gravity of the Moon
&	The statement continues on the next line.	x = a + b + &

Note to the translators (8) · A structure word, for example PROGRAM, MODULE or CONTAINS.

Word	Meaning	Example
PROGRAM	The start of the primary program.	program lander
END	The end of a program, module, subroutine or function.	end program lander
MODULE	A named box of related names and procedures.	module physics
USE	Make the names in a module available here.	use physics
CONTAINS	The procedures come after this line.	contains
SUBROUTINE	A named procedure that does a job.	subroutine draw(ship)
FUNCTION	A named procedure that returns one value.	function speed(v)
IMPLICIT NONE	Every name must have a declared type. Without it, old Fortran guesses the type from the first letter.	implicit none

Watch out: what the review found

These notes come from the review of the AI-written guides. They show traps that are easy to miss.

• D01-M5 major In practice this is false. GFortran coarrays need OpenCoarrays, built on MPI (a third-party message-passing library). The Intel implementation uses Intel MPI. MPI+OpenMP dominates production HPC (high-performance computing), so coarrays are niche. The plan puts "distributed memory scaling via coarrays" in weeks 9 to 12, and that goal is aspirational.
• D01-M7 major LFortran was still pre-1.0 (alpha/beta) at the date of the cited sources. The document recommends it to a 90-day learner as equal to Python/Julia interactivity. That overstates its maturity.
• D15-C4 critical The 5A22 has a hardware multiplier at $4202/$4203 (8 x 8, result at $4216/$4217, 8 cycles) and a divider at $4204 to $4206 (16 / 8, quotient at $4214/$4215). The reviewer fetched the nesdev "Multiplication" page for the multiplier. A binary to decimal conversion is a few hardware divisions, not 320 cycles. The video says only that BCD "makes the math so much easier" [31:44] and that INC and DEC ignore the decimal flag [32:05]. None of the four chart values is in the video or in report 14. The chart is a picture of an invention.

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