Address constant
In IBM/360 (and through to present day z/Architecture), an address constant or "adcon" is an Assembly language data type whose value refers directly to (or "points to") another value stored elsewhere in the computer memory using its address. An address constant can be one, two, three or four bytes long (for IBM/360 architecture). It is defined using an assembler language "DC" statement using a type of A (or V if the adcon refers to an address outside of the current program module).
If the adcon is less than three bytes it is usually used to hold a 16bit integer such as a length, a relative address or some index value. If the adcon is a 'V' type, it addresses an external program entry point, resolved by the link-editor when the external module is included with the module making the reference.
IBM S/360 and z/Architecture Assembler example
All these are valid adcon's:-
ADCONS DS 0A an aligned label of implicit length 4 (for the next 4 byte address that follows it)
DC A(FIELDA) a 4 byte word, aligned, absolute address of a variable 'FIELDA'
DC AL4(FIELDA) as above but not (necessarily) aligned on a word boundary
DC AL3(FIELDA) a three byte equivalent of the above (maximum 16 megabytes)
DC AL2(FIELDA-TABLES) two byte offset from 'TABLES' label to start of 'FIELDA'
DC AL2(L'FIELDA) a two byte length of the field called 'FIELDA' (=26 in decimal)
DC AL1(C'A') hexadecimal value of the EBCDIC character 'A' (=C1 in hex)
DC A(FIELDA-C'A') a 4 byte, aligned, absolute address --> 192 bytes before the start of FIELDA
DC A(*) a 4 byte, aligned, address of this adcon (* means 'here')
INDIRECT DC A(*+4) address of next byte after this adcon (the V-type adcon)
DC V(SUBRTNX) address of an external subroutine entry point
DC AL1(-1) a one byte negative value (= x'FF'), often used as a table de-limiter
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SUBRTNA DS 0H start of (internal) sub-routine A
. instructions go here
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TABLES DS 0H base address for tables section (halfword aligned)
LENGTHS DC Al2(5,27,56,83,127,32563) an arbitrary array of 6 x 2 byte hex lengths (defined by their decimal values)
PARMLIST DC A(HERE,THERE,EVWHERE,-1) an array of 3 x 4 byte aligned pointers to various field labels/entrypoints with additional negative value end-pointer (=X'FFFFFFFF').
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ZERO_255 DC 256AL1(ZERO_255-*) an array of 256 single byte hex values 00-FF
* ---------end of adcon examples ---------------- *
FIELDA DC C'ABCDEFGHIJKLMNOPQRSTUVWXYZ' a field containing a character string (not an adcon) = A-Z
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