There are several ways for programs to change their paths of execution,
what would now be done by setting and testing boolean or logical
Word marks as flags
Word marks can be set or cleared with four-character instructions, and
tested with an eight-character instruction.
sw flag turn on the branch
cw flag turn off the branch
bw xyz,flag branch to xyz if flag has a word mark
flag dcw #1
Characters as flags
Characters can be used as flags, but this is somewhat less efficient
than using word marks, since seven-character instructions are needed to
change them, and a value to put into them has to be stored somewhere.
An advantage is that they can have more than two values.
mcw @x@,flag turn on the branch
mcw @y@,flag turn off the branch, maybe turn a different one on
mcw @z@,flag turn off the branch, maybe turn a different one on
bce xyz,flag,x branch to xyz if flag is x
flag dcw #1
Modify the OP code
The op code can be changed between branch and NOP
mcw branch,switch turn on the branch
mcw nop,switch turn off the branch
switch b xyz branch if the OP code is not NOP
The values to set into the OP code can be ordinary literal values, or OP
codes of instructions that don't change. A program always has branches
for other purposes. NOP is less commonly needed.
Switch branch with word marks
This is the most efficient method. Branches can be switched by setting
and clearing word marks, either at the OP code of the D-modifier.
Initially on branch
An initially on branch is implemented by a NOP followed by a branch.
The branch is switched by setting or clearing the word mark under its OP
switch b xyz branch if switch has a word mark, else part of NOP
cw switch turn off the branch
sw switch turn it back on
Initially off branch
An initially off branch is implemented as a branch-on-indicator using an
indicator that is never set by the processor, such as a group mark. The
branch is switched by setting or clearing the word mark under the
switch bin xyz,} branch if D-modifier has a word mark
cw switch&4 turn off the branch
sw switch&4 turn on the branch