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+.NH 2
+Parameters and local variables.
+.PP
+In the EM calling sequence, the calling procedure
+pushes its parameters on the stack
+before doing the CAL.
+The called routine first saves some
+status information on the stack and then
+allocates space for its own locals
+(also on the stack).
+Usually, one special purpose register,
+the Local Base (LB) register,
+is used to access both the locals and the
+parameters.
+If memory is highly segmented,
+the stack frames of the caller and the callee
+may be allocated in different fragments;
+an extra Argument Base (AB) register is used
+in this case to access the actual parameters.
+See 4.2 of
+.[
+keizer architecture
+.]
+for further details.
+.PP
+If a procedure call is expanded in line,
+there are two problems:
+.IP 1. 3
+No stack frame will be allocated for the called procedure;
+we must find another place to put its locals.
+.IP 2.
+The LB register cannot be used to access the actual
+parameters;
+as the CAL instruction is deleted, the LB will
+still point to the local base of the \fIcalling\fR procedure.
+.LP
+The local variables of the called procedure will
+be put in the stack frame of the calling procedure,
+just after its own locals.
+The size of the stack frame of the
+calling procedure will be increased
+during its entire lifetime.
+Therefore our model will allow a
+limit to be set on the number of bytes
+for locals that the called procedure may have
+(see next section).
+.PP
+There are several alternatives to access the parameters.
+An actual parameter may be any auxiliary expression,
+which we will refer to as
+the \fIactual parameter expression\fR.
+The value of this expression is stored
+in a location on the stack (see above),
+the \fIparameter location\fR.
+.sp 0
+The alternatives for accessing parameters are:
+.IP -
+save the value of the stackpointer at the point of the CAL
+in a temporary variable X;
+this variable can be used to simulate the AB register,  i.e.
+parameter locations are accessed via an offset to
+the value of X.
+.IP -
+create a new temporary local variable T for
+the parameter (in the stack frame of the caller);
+every access to the parameter location must be changed
+into an access to T.
+.IP -
+do not evaluate the actual parameter expression before the call;
+instead, substitute this expression for every use of the
+parameter location.
+.LP
+The first method may be expensive if X is not
+put in a register.
+We will not use this method.
+The time required to evaluate and access the
+parameters when the second method is used
+will not differ much from the normal
+calling sequence (i.e. not in line call).
+It is not expensive, but there are no
+extra savings either.
+The third method is essentially the 'by name'
+parameter mechanism of Algol60.
+If the actual parameter is just a numeric constant,
+it is advantageous to use it.
+Yet, there are several circumstances
+under which it cannot or should not be used.
+We will deal with this in the next section.
+.sp 0
+In general we will use the third method,
+if it is possible and desirable.
+Such parameters will be called \fIin line parameters\fR.
+In all other cases we will use the second method.