To call an assembly (or machine) language subroutine.
numvar is the starting point in memory of the subroutine being called as an offset into the current segment.
variables are the variables or constants, separated by commas and enclosed in parentheses, that are to be passed to the routine.
The CALL statement is recommended for interfacing assembly language programs with GW-BASIC. Although the USR function may also be used, CALL is compatible with more languages, produces a more readable source code, and can pass multiple arguments.
Invocation of the CALL statement causes the following to occur:
Each parameter location in the variable is pushed onto the stack. The parameter location is a 2-byte offset into GW-BASIC's data segment.
The return address code segment (CS) and the offset are pushed onto the stack.
Control is transferred to the user routine by the segment address given in the last DEF SEG statement and the offset given in the variable name.
The user routine now has control. Parameters may be referenced by moving the stack pointer (SP) to the base pointer (BP) and adding a positive offset to BP.
The called routine may destroy the contents of any registers.
The called program must know how many parameters were passed. Parameters are referenced by adding a positive offset to BP, assuming the called routine moved the current stack pointer into BP (that is, MOV BP,SP).
The called program must know the variable type for numeric parameters passed.
The called routine must do a RET n, where n is the number of parameters in the variable times 2. This is necessary in order to adjust the stack to the point at the start of the calling sequence.
Values are returned to GW-BASIC by including in the argument list the name of the variable that is to receive the result.
If the argument is a string, the parameter offset points to three bytes called the string descriptor. Byte 0 of the string descriptor contains the length of the string (0 to 255). Bytes 1 and 2, respectively, are the lower- and upper-eight bits of the string starting address in the string space.
If the argument is a string literal in the program, the string descriptor points to program text. Be careful not to alter or destroy a program this way. To avoid unpredictable results, add +"" to the string literal in the program, as in the following:
This forces the string literal to be copied into the string space. Now the string may be modified without affecting the program.
Strings may be altered by user routines, but their length must not be changed. GW-BASIC cannot correctly erase strings if their lengths are modified by external routines.
Line 100 sets the segment to hex 2000. ARK is set to zero so that the call to ARK executes the subroutine at location 2000:0.
The following sequence of 8086 Assembly Language demonstrates access of the parameters passed and stored in variable C:
MOV BP, SP ; Gets current stack position in BP.
MOV BX, 8[BP] ; Gets address of B$ descriptor.
MOV CL, [BX] ; Gets length of B$ in CL.
MOV DX, 1[BX] ; Gets address of B$ text in DX.
MOV SI, 10[BP] ; Gets address of A in SI.
MOV DI, 6[BP] ; Gets pointer to C in DI.
MOVSW ; Stores variable A in C.
RET 6 ; Restores stack and returns.
MOVSW copies only two bytes. This is sufficient if variables A and C are integer. Four bytes must be copied if they are single precision; eight bytes, if they are double precision.
Line 100 sets the segment to hex 2000. The value of variable ACC is added into the address as the low word after the DEF SEG value is shifted four bits to the left (this is a function of the microprocessor, not of GW-BASIC). Here, ACC is set to &H7FA, so that the call to ACC executes the subroutine at the location hex 2000:7FA (absolute address hex 207FA).
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