User guide

48 CHAPTER 3. THE LIBRARY

next resumes execution. The parent link points to the coroutine that called this one,

or is zero if the coroutine not active. The outermost corouti ne (or root coro u ti ne) is

marked by the special value -1 in its parent link. As a debugging aid, all coroutines

are chained together in a list held in the global colist. The values fn and sz hold the

main function of the coroutine and its stack size, and c is a private variabl e used by

the coroutine mechanism.

changeco

CHGCOP

aL1P1

P1 L1 a

currco PPC

stack frame

currco

resumption point

cptr

Figure 3.2: The eﬀect of changeco(a, cptr)

At any time just one coroutine (the current coroutine) has control, and all the

others are said to be suspended. The current coroutine is held in the global variable

currco, and the Cintcode P register points to a stack frame within its stack. Passing

control from one coroutine to another involves saving the resumption point in the

current coroutine, and setting new values for the program counter (PC), the P pointer

and currco. This is done by changeco(a,cptr) as shown in ﬁgure 3.2. The functi on

changeco is deﬁned by hand in syslib used by cintsys and cintpos and its body

consists of the single Cintcode instruction CHGCO. As can be seen its eﬀect is somewhat

subtle. The only uses of changeco are in the deﬁnitions of createco, callco, cowait

and resumeco, and these are the only functions that cause coroutine suspension. In

the native code version of BCPL changeco is de ﬁn ed in mlib.s