Global Unique:

Z%  : boxed memory values
Z1  : Z% size
Z2  : S$ size
Z3  : stack start address (cbm) or X% size (qbasic)
Z4  : release stack start address (cbm) or Y% size (qbasic)
ZI  : start of unused memory (index into Z%)
ZK  : start of free list (index into Z%)
ZT  : top of memory after repl env allocations

S$  : string memory storage
S   : next free index in S$

X%  : logic/call stack (Z% indexes)
X   : top element of X% stack

Y%  : pending release stack [index into Z%, eval level]
Y   : top element of Y% stack

D   : root repl environment

BT  : begin time (TI)
ER  : error type (-2: none, -1: string, >=0: object)
E$  : error string (ER=-1)
EZ  : READLINE EOF return, READ_FILE EOF temp

LV  : EVAL stack call level/depth

RI  : reader current string position
RJ  : READ_TOKEN current character index


Calling arguments/temporaries:

A   : common call argument (especially EVAL, EVAL_AST)
A$  : common call argument (READLINE, reader, string temp, key value)
B   : common call argument
B$  : STRING arg (HASHMAP_GET temp), PR_STR_SEQ separator
    : INIT_CORE_SET_FUNCTION, ENV_SET_S, ASSOC1_S
C   : common call argument, DO_TCO_FUNCTION temp in DO_APPLY
E   : environment (EVAL, EVAL_AST)
F   : function
H   : hash map
K   : hash map key (Z% index)
L   : ALLOC* Z%(R,1) default
M   : ALLOC* Z%(R+1,0) default
N   : ALLOC* Z%(R+1,1) default
R   : common return value
R$  : common string return value
T   : type arg, common temp
Q   : PUSH*, POP*, PEEK* return value (and PEEK_Q_Q call arg)

AR  : APPLY, DO_*_FUNCTION arg list
AY  : RELEASE/FREE arg
AZ  : PR_STR arg
P1  : PR_MEMORY*, PR_OBJECT, CHECK_FREE_LIST start
P2  : PR_MEMORY*, PR_OBJECT, CHECK_FREE_LIST end
P3  : PR_OBJECT, PR_MEMORY_VALUE
R1  : REP, RE - MAL_READ result temp
R2  : REP, RE - EVAL result temp
R3  : HASHMAP_GET, DO_HASH_MAP, DO_KEYS_VALS temp and return value
R4  : ENV_FIND temp and return value
R6  : SLICE return value (last element)
SZ  : size argument to ALLOC
S1$ : REPLACE needle
S2$ : REPLACE replacement


Other temporaries:

A0  : EVAL ast elements
A1  : EVAL ast elements, DO_FUNCTION temp
A2  : EVAL ast elements, DO_FUNCTION temp
A3  : EVAL ast elements
B1  : DO_FUNCTION temp

CZ  : DO_CONCAT stack position
ED  : EQUAL_Q recursion depth counter
RD  : PR_OBJECT recursion depth
SD  : READ_STR sequence read recursion depth

C$  : READ_TOKEN, SKIP_SPACES, SKIP_TO_EOL current character
D$  : READ_TOKEN/READ_FILE_CHAR temp
G   : function value ON GOTO switch flag, EVAL_AST changed flag
I   : STRING, REPLACE, SLICE, PR_MEMORY, PR_OBJECT, PR_MEMORY_VALUE
J   : REPLACE, PR_MEMORY_VALUE
U   : ALLOC, RELEASE, PR_STR temp
V   : RELEASE, PR_STR_SEQ temp
W   : SLICE, LAST, QUASIQUOTE, DO_HASH_MAP, DO_KEYS_VALS, step2-3 EVAL temp
P   : PR_MEMORY_SUMMARY_SMALL
RC  : RELEASE remaining number of elements to release
RF  : reader reading from file flag
S1  : READ_TOKEN in a string?
S2  : READ_TOKEN escaped?
T$  : READ_* current token string
T1  : EQUAL_Q, PR_STR, DO_KEYS_VALS temp
T2  : EQUAL_Q, DO_KEY_VALS, HASH_MAP_GET
T3$ : REPLACE temp


Unused:

O


Counting number of times each variable is assigned:
	sed 's/:/\n /g' readline.in.bas types.in.bas reader.in.bas printer.in.bas env.in.bas core.in.bas stepA_mal.in.bas | grep "[A-Z][A-Z0-9]*[%$]*=" | sed 's/.*[^A-Z]\([A-Z][A-Z0-9]*[%$]*\)=.*/\1/g' | sort | uniq -c | sort -n