User guide

3.3. GLOBAL FUNCTIONS 57

destination task or device (speciﬁed by pkt

id!pkt). If this ﬁel d is positive it refer s to

a task, if it is -1 it r e fe r s to the clock device and other negative val ues refer to other

devices. If the packet is queued successfully this ﬁeld is updated to hold the current

task’s identiﬁer and the result i s non ze r o, otherwise the result is zero with result2

set to 101 if the destination id is invalid, and to 111 if pkt link was not equal to

notinuse (=-1). If the destination was a runnable task of higher priority than the

current one, th en the current task immediately becomes suspended in RUN state and

control is given to the destination, otherwise the curr ent task continues to r un normally.

Interaction with the resident Cintpos devices is described i n Chapter 6.

n := randno(upb) CIN:y, POS:y, NAT:y

This function returns a random integer in the range 1 to upb. It uses a seed held

in global variable randseed which can be set using setseed described below. Its

implementation is as follows:

LET randno(upb) = VALOF

{ randseed := randseed*2147001325 + 715136305

RETURN ABS(randseed/3) REM upb + 1

}

res := rdargs(keys, argv, upb) CIN:y, POS:y, NAT:y

This impl e me ntation of BCPL incorporates a command language interpreter which

is described in Chapter 4. Most commands require arguments and these are easily read

using rdargs.

The ﬁrst argument (keys) speciﬁes the argument format. The second and third

arguments provide a vector (argv) with a given upper bound (upb) into which the

decoded arguments will be placed. If rdargs is succ es sfu l , it returns the number of

words used in a rgv to represent the decoded command arguments, but on failure, it

returns zero.

The stri ng keys holds the list of argument keywords separated by commas (,).

Alternative keywords for a given argument are separated by equal signs (=). The

expected number of arguments is one more than the number of commas in the key

string. If rdargs returns successfully, this number of elements at the start of argv wil l

hold the decoded arguments.

Arguments can have qualiﬁers of the form /A, /K, /N, /S and /P. The qualiﬁer

letters can be in either upper or lower case. The qualiﬁer /A means that the argument

must b e given. /K means that, if the argument is given, it must include its keyword.

/N speciﬁes that the argument must be a number. /S indicates that the argument is

a switch parameter se t to TRUE by its keyword. /P indicates that a prompt will be

given for the argument if it has not already b ee n set. Pr omp ti ng only happens if the

currently selected input and output streams are connected to an interactive terminal.

If the pr ompt is for a switch argument (/S) it expects a yes/no response. Typing

yes or y is treated as yes, any other response is treated as no. If rdargs re t ur ns

successfully argv!0, argv!1 etc will hold the arguments settings. A setting of zero

means the argument was not given. A setting of -1 means the argument was a switch

set the TRUE. Othe r wis e , if /N was speciﬁed the setting will point to a word in argv