Global Unique:

Z%  : boxed memory values
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

ER  : error type (-2: none, -1: string, >=0: object)
E$  : error string (ER=-1)
EZ  : READLINE EOF

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)
C   : common call argument
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

AY  : RELEASE/FREE arg
AZ  : PR_STR arg
P1  : PR_MEMORY, CHECK_FREE_LIST start
P2  : PR_MEMORY, CHECK_FREE_LIST end
SZ  : size argument to ALLOC
S1$ : REPLACE needle
S2$ : REPLACE replacement

Other temporaries:
A0  : EVAL ast elements
A1  : EVAL ast elements
A2  : EVAL ast elements
A3  : EVAL ast elements

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
G   : function value ON GOTO switch flag
I   : STRING, REPLACE, SLICE, PR_MEMORY, PR_OBJECT
J   : REPLACE
S1  : READ_TOKEN in a string?
S2  : READ_TOKEN escaped?
T$  : READ_* current token string
T1  : EQUAL_Q, PR_STR, and core DO_KEYS_VALS temp
T2  : EQUAL_Q, DO_KEY_VALS, HASH_MAP_GET
T3  : HASHMAP_GET temp and return value
T3$ : REPLACE temp
T4  : ENV_FIND temp and return value
T6  : LAST and QUASIQUOTE temp
T7  : READ_FORM temp
T8  : READ_FORM_DONE temp
T9  : PR_STR_SEQ temp
U3  : ALLOC
U4  : ALLOC
U6  : RELEASE
U7  : RELEASE

Unused:

O, Q, U, V, W