small LOAD cleanup

c64/kernal/iostack/f4a5.load.s

@@ -0,0 +1,54 @@
+//
+// Official Kernal routine, described in:
+//
+// - [RG64] C64 Programmer's Reference Guide   - page 286
+// - [CM64] Compute's Mapping the Commodore 64 - page 231
+// - IEC reference at http://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
+//
+// CPU registers that has to be preserved (see [RG64]): none
+//
+
+
+	// Expects that SETLFS and SETNAM are called before hand.
+	// $YYXX = load address.
+	// (ignored if SETLFS channel = 1, i.e., like ,8,1)
+	// If A=1 then VERIFY instead of LOAD.
+	// On exit, $YYXX is the highest address into which data
+	// will have been placed.
+
+	// XXX add VERIFY support
+
+LOAD:
+
+	// Are we loading or verifying?
+	sta VERCKK
+
+	// Store start address of LOAD
+	lda MEMUSS+0
+	sta STAL+0
+	lda MEMUSS+1
+	sty STAL+1
+
+	// Reset status
+	jsr kernalstatus_reset
+
+#if CONFIG_MEMORY_MODEL_60K
+	// We need our helpers to get to filenames under ROMs or IO area
+	jsr install_ram_routines
+#endif
+
+	// Check whether we support the requested device
+	lda FA
+
+	// Try IEC - legal device numbers for LOAD are 8-30
+	// Device numbers above 30 can not be used as IEC (see https://www.pagetable.com/?p=1031),
+	// as the protocol combines device number with command code in one byte, above 30 it is
+	// no longer possible. Original ROMs show ILLEGAL DEVICE NUMBER error only for devices
+	// 0-3; our implementation is more strict here
+
+	cmp #$1F
+	bcs !+              // >= 31  
+	cmp #$08
+	bcs_far load_iec    // >= 8
+!:
+	jmp lvs_illegal_device_number