OS_Fn, Handle functions

Register parameters

RST 20H, DEFB $7B

IN:
     A = reason code
     Other register parameters depend on reason code
OUT:
     Depends on reason code, see below.

Notes

Internal operating system usage. This call is used to allocate all I/O type handles. The handle type and structure is documented in handle.def file.

FN_AH (A = $01), Allocate handle

IN:  A = FN_AH ($01)
	 B = handle type
     B = 0, returns the first free handle, does not allocate it properly

OUT, if successful:
     Fc = 0, IX = handle

OUT, if failed:
     Fc = 1, A = RC_ROOM

Registers changed after return:
     ..BCDEHL/..IY same
     AF....../IX.. different

FN_VH (A = $02), Verify handle

IN:  A = FN_VH ($02)
     B = handle type
     IX = handle

OUT, if successful:
     Fc = 0

OUT, if failed
     Fc = 1, A = RC_ROOM

Registers changed after return:
     ..BCDEHL/IXIY same
     AF....../.... different

FN_FH (A = $03), Free handle

IN:  A = FN_FH ($03)
     B = handle type
     IX = handle

OUT, if successful:
     Fc = 0, IX = 0

OUT, if failed:
     Fc = 1, A = RC_HAND

Registers changed after return:
     ..BCDEHL/..IY same
     AF....../IX.. different

FN_GH (A = $04), Get (Find) handle (OZ 4.6 and newer)

IN:  A = FN_GH ($04)
     IX = first handle to test, 0 to begin with top handle
     B = handle type, 0 for any type
     C = dyn id, 0 for any dyn id

OUT, if successful:
     Fc = 0, IX = handle

OUT, if failed:
     Fc = 1, A = RC_EOF

Registers changed after return:
     ..BCDEHL/..IY same
     AF....../IX.. different

FN_CH (A = $05), Change handle type (0Z 5.0 and newer)

IN:  A = FN_CH ($05)
	 B = old type
	 C = new type
     IX = handle
OUT, if successful:
     Fc = 0, IX = same handle
OUT, if failed:
     Fc = 1, A = RC_Hand (old type wrong)

Registers changed after return:
     A.BCDEHL/IXIY same
     .F....../.... different