On Mon, 22 Jun 2026, Paul Koning via cctalk wrote:
I also remember running into him late one night, in the middle of a
debug session. I asked what he was working on. Answer "packed decimal
exponentiation". My question "what is that used for?" Answer:
"compound interest in COBOL programming".
Oh yeah.
That's what it is for
In beginning high level language programming classes (FORTRAN, BASIC, C),
I often gave compound interest as an early exercise in loops (displaying
each iteration, not exponential direct calculation of end result, not
BCD)
But, I used BCD a little in my sales tax TSR.
And, I have often [mis]used x86 BCD instructions for display of numbers:
AAM as a substitute for dividing by 10 for 0-99 two decimal digit number
in an 8 bit register,
and DAA in converting a single hexadecimal digit into ASCII numeral: (notice
ADC to avoid JMPs)
AND AL, 0Fh
DAA
ADD AL, 0F0h
ADC AL, 40h
or:
AND AL, 0Fh
ADD AL, 0
ADC AL,28h
DAA
That is extendable to full content (4 hex digits) of 16 bit AX register:
PUSH CX ; in case CX is in use outside of this
MOV CX, 0404h ; CH and CL are independent; CL as a ROL loop, CH as a
manual count down
N1: ROL AX, CL
PUSH AX ;gonna get stepped on in display output
AND AL, 0Fh ;disply single hexadecimal digit
DAA
ADD AL, 0F0h
ADC AL, 40h
MOV AH, 0Eh ;displays char in AL at cursor position
INT 10h ; or save registers and use DL, and INT21h display
POP AX ;bring it back after display
DEC CH
JNZ N1
POP CX ; put it back
