HP HP-16C - Section 6: Memory and Storage; Viewing the Status of Memory Allocation ([MEM])

Section 6: Memory and Storage 63
Therefore, the number of bytes dedicated to storage registers and
the number of bytes dedicated to programming are always
multiples of seven.
Note: The calculator converts data storage registers to
program memory in reverse numerical order, from the
highest numbered to the lowest numbered. Furthermore,
any data contained in a storage register will be lost when
that register is converted to lines of program memory.
Converting Program Memory to Storage Registers
Once you have stored program instructions they are well protected.
The allocation of program memory space back to data storage
registers 1s accomplished only by your intentionally deleting
program instructions—either singly or all at once (by CLEAR
or Continuous Memory reset). What is deleted from program
memory is reallocated to data storage memory in increments of
seven bytes.
Note that this means you cannot unintentionally lose program
instructions. If you try to address a storage register whose space is
occupied by program lines, an Error3 (nonexistent storage
register) will result.
Storage Register Size
Each register represents a word, and so its size depends on the
current word size.* The size of a storage register is always the
smallest multiple of four bits (half-bytes) equal to or greater than
the current word size. For example, a current word size of either 13,
14, 15, or 16 will produce a storage register length of 16 bits (two
bytes).
In Floating-Point Decimal mode, the word size and therefore the
storage register size are automatically set to 56 bits (7 bytes).
The total possible number of data storage registers equals the
number of available bytes (203 minus the bytes of program
memory) divided by the number of 8-bit bytes per register. For
example, if the current word size is 16 bits, then each register
comprises 2 bytes (16 bits). If program memory is cleared (203 bytes
* Except the Index register (discussed later in this section), which is of constant size (68
bits) and not convertible to program memory.