Vector Basic Interpreter Basic 80, Справочное руководство, Страница: 157 / 190

Extended and Disk BASIC-80 user
made' with the CALL statement.
the same as that in Microsoft's
compilers.
A CALL statement with no arguments generates a simple "CALL"
instruction. The corresponding subroutine should return via
a simple "RET." (CALL and RET are 8080 opcodes - see an 8080
reference manual for details.)
function calls may also be
The calling sequence used is
FORTRAN, COBOL and BASIC
A subroutine CALL with arguments results in a somewhat more
complex calling sequence. For. each argument in the CALL
argument list, a parameter is passed to the subroutine.
That parameter is the address of the low byte of the
argument. Therefore, parameters always occupy two bytes
each,regardless of type.
The method of passing the parameters depends upon the number
of parameters to pass:
1. If the number of parameters is less than or equal
to 3, they are passed in the registers. Parameter
1 will be in aL, 2 in DE (if present), and 3 in BC
(if present).
2. If the number of parameters is greater than 3, they
are passed as follows:
3. Parameters 3 through n in a contiguous data
block. BC will point to the low byte of this
data block (i.e., to the low byte of parameter
3) •
Note that, with this scheme, the subroutine must know how
many parameters to expect in order to find them.
Conversely, the calling program is responsible for passing
the correct number of parameters. There are no checks for
correct number or type of parameters.
If the subroutine expects more than 3 parameters, and needs
to transfer them to a local data area, there is a system
subroutine which will perform this transfer. This argument
transfer routine is named $AT (located in the FORT~~
library, FORLIB.REL), and is called with HL pointing to the
local data area, BC pointing to the third parameter, and A
containing the number of arguments to transfer (i.e., the
total number of arguments minus 2). The subroutine is