Texas Instruments TI Programmable 57 Manual page 45

There is a family of instructions on your calculator designed to let you
build decision-making into your programs. The basic operation of this
family of instructions is similar — and they're quite easy to learn, but they
are also flexible enough to allow a great deal of programming power.
Basically when a conditional transfer key sequence is encountered in a
program, the calculator immediately compares two quantities: the
number in the display register (called x) and the number in a special test
or "t" register (called t). (The "'t' or test register is just memory 70on your
calculator.) The calculator then asks one of 4 questions about how x and
t compare — depending on the program instruction you use:
EM isx = t? (Isx equal t?)
(NV) (2nd) BH = Isx # t? (Is x not equal t?)
(2né) EM = Isx => t? (Is x greater than or.equal to t?)
(iV) (2nd) EI = Isx < t? (Isx less than t?)
If the answer is yes, the program pointer goes on to the instruction
immediately following. If the answer is NO, the program pointer SKIPS the
instruction immediately following and continues.
Let's go through each of the conditional transfer instructions (and the
keys for using them) one at a time.
[xst] — THE "x EXCHANGE wiTH t" Key
When this key is pressed the contents of the display register and the "'t''
register (memory 7) are just exchanged. This provides a convenient
method of getting any test, or t, value into the test register.
(You can also use the @ 7 key sequence, or simply the 7
sequence if it is appropriate in your program.)