Defining and using functions and instructions

The following instructions make it possible to define and use both functions and new instructions.

In addition to a simple call function, there is a loop and call function.

An IDEF or instruction definition call makes it possible to patch old scalers in order to add newly defined instructions.

Function DEFinition

FDEF[ ]

Code Range	0x2C
Pops	f: function identifier number (integer in the range 0 to n-1 where n is specified in the 'maxp' table )
Pushes	–

Marks the start of a function definition. The argument f is a number that uniquely identifies this function. A function definition can appear only in the Font Program or the CVT program. Functions may not exceed 64K in size.

END Function definition

ENDF[ ]

Code Range	0x2D
Pops	–
Pushes	–

Marks the end of a function definition or an instruction definition.

CALL function

CALL[ ]

Code Range	0x2B
Pops	f: function identifier number (integer in the range 0 to n-1 where n is specified in the 'maxp' table )
Pushes	–

Calls the function identified by the number f.

LOOP and CALL function

LOOPCALL[ ]

Code Range	0x2A
Pops	f: function number integer in the range 0 to n-1 where n is specified in the 'maxp' table
	count: number of times to call the function (signed word)
Pushes	–

Calls the function f, count number of times.

Example:

Assume the Font Program contains this:

PUSHB(000), 17	push 17 onto the stack
FDEF[ ]	start defining function 17
...	contents of function
...
ENDF[ ]	end the definition
PUSHB(000),17	push function number
PUSHB(000),5	push count
LOOPCALL[ ]	call function17, 5 times

Instruction DEFinition

IDEF[ ]

Code Range	0x89
Pops	opcode (8 bit code padded with zeroes to ULONG)
Pushes	–

Begins the definition of an instruction. The instruction definition terminates when at ENDF, which is encountered in the instruction stream. Subsequent executions of the opcode popped will be directed to the contents of this instruction definition (IDEF). IDEFs should be defined in the Font Program or the CVT Program. An IDEF affects only undefined opcodes. If the opcode in question is already defined, the interpreter will ignore the IDEF. This is to be used as a patching mechanism for future instructions. Instructions may not exceed 64K in size.