tape wedge can now invoke MERGE, documentation update

src/basic/wedge/wedge_tape_prepare_load.s

@@ -0,0 +1,68 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+
+#if CONFIG_TAPE_WEDGE
+
+//
+// Common part for LOAD and MERGE operations via tape wedge
+//
+
+wedge_tape_prepare_load:
+
+	// Make sure the syntax is correct
+
+	jsr injest_spaces
+	jsr fetch_character
+	
+	cmp #$00
+	beq wedge_tape_prepare_load_no_filename      // branch if no file name given
+	cmp #$22
+	bne_16 do_SYNTAX_error                       // branch if no opening quote
+
+	// Fetch the file name
+
+	lda TXTPTR+0
+	sta FNADDR+0
+	lda TXTPTR+1
+	sta FNADDR+1
+
+	ldx #$00
+!:
+	jsr fetch_character
+
+	cmp #$00
+	beq !+
+	cmp #$22
+	beq !+
+
+	inx
+	bne !-
+!:
+	stx FNLEN
+
+	lda #$00
+	beq wedge_tape_prepare_load_got_filename
+
+wedge_tape_prepare_load_no_filename:
+	
+	sta FNLEN                                    // .A is 0 here, default file name is empty
+	
+	// FALLTROUGH
+
+wedge_tape_prepare_load_got_filename:            // .A has to be 0
+
+	sta VERCKB                                   // operation is LOAD, not VERIFY
+
+	ldy #$01
+#if CONFIG_TAPE_TURBO
+	ldx #$07                                     // turbo tape device
+#else
+	ldx #$01                                     // normal tape device
+#endif
+
+	jmp JSETFLS
+
+
+#endif // CONFIG_TAPE_WEDGE