/usr/share/doc/nasm/examples/dtbcd.asm is in nasm 2.11.08-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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; This is a macro to generate packed BCD constants.
; It is obsoleted by "dt" with a packed BCD value with a "p"
; suffix, but it is included here as a rest of nested %rep.
;
%macro dtbcd 1-*.nolist
%push dtbcd
%rep %0
%defstr %$abc %1
%substr %$sign %$abc 1
%if %$sign == '-'
%substr %$abc %$abc 2,-1
%xdefine %$sign 0x80
%elif %$sign == '+'
%substr %$abc %$abc 2,-1
%xdefine %$sign 0x00
%else
%xdefine %$sign 0x00
%endif
%strlen %$abclen %$abc
%defstr %$abclen_str %$abclen
%assign %$pos %$abclen
%assign %$bc 0
%assign %$ld -1
%rep %$abclen
%substr %$chr %$abc %$pos
%assign %$pos %$pos-1
%if %$chr >= '0' && %$chr <= '9'
%if %$ld < 0
%assign %$ld %$chr-'0'
%assign %$bc %$bc+1
%if %$bc > 9
%warning "too many digits in BCD constant"
%exitrep
%endif
%else
db %$ld+((%$chr-'0') << 4)
%assign %$ld -1
%endif
%elif %$chr == '_'
; Do nothing...
%else
%error "invalid character in BCD constant"
%exitrep
%endif
%endrep
%if %$ld >= 0
db %$ld
%endif
%rep 9-%$bc
db 0
%endrep
db %$sign
%rotate 1
%endrep
%pop
%endmacro
dtbcd 123, -456, +789
dt 123p, -456p, +789p
dtbcd 765432109876543210
dt 765432109876543210p
dtbcd -765432109876543210
dt -765432109876543210p
dtbcd +765_432_109_876_543_210
dt +765_432_109_876_543_210p
dtbcd -765_432_109_876_543_210
dt -765_432_109_876_543_210p
;; Both of these should warn...
dtbcd 8_765_432_109_876_543_210
dt 8_765_432_109_876_543_210p
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