The OBSINP commands that reference global memory (e.g. STR=) use a single
letter plus a number to specify which memory location is to be referenced.
The number is an offset (REAL/INTEGER *4 units) to add to the address to 
which the single letter refers.  At the HHT, the following letters are
defined and reference the variable specified:

Letter	Variable	Description
A	BAS_INPUT(1)	First word in /YY_PASSIV/
N	NAME_LENGTH	Object name; variable after FE_PASSIV() in /YY_PASSIV/
R	ICOUNTERS	SRPn counters in /YY_PASSIV/
S	ISTORE		Corresponds to OLAM; in /SCREEN/ (OBSINP common)
C	LCATALOG	OBSINP catalog name; in /SCREEN/
T	TIME_INPUT	STIM; in /YY_PASSIV/
D	DP_PASSIV	Data Proc. passive field. D% = beg. of current DPn (128)
B	BE_PASSIV	Backend passive field.  B% = beg. of current BEn field
F	FE_PASSIV	Frontend passive field.  F% = beg. of current FEn field.
I	ISUB_START	SSUB; in /YY_PASSIV/
Q	PONT_ACTIVE	NULE currently in effect; in /YY_PASSIV/
M	MORE_PASSIVE	Space for additional passive variables; in /YY_PASSIV/
U	BE_SAVE		(Semi)active field BE area (as B, above); in /OLDPASSIV/
V	FE_SAVE		(Semi)active field FE area (as F, above); in /OLDPASSIV/