implemented MERGE command

c64/basic/basic_commands_CC/cmd_merge.s

@@ -5,4 +5,46 @@
 
 cmd_merge:
 
-	jmp do_NOT_IMPLEMENTED_error // XXX implement the command
+	lda #$00                           // mark operation as LOAD
+	sta VERCKB
+
+	// Set default device and secondary address; channel is not important now
+
+	jsr select_device
+	ldy #$00                           // secondary address 0 - it has to be loaded precisely where we want
+	jsr JSETFLS
+
+	// Fetch the file name
+
+	jsr fetch_filename
+	bcc !+
+
+	// No filename supplied - this should only be allowed for tape (device number below 8)
+
+	lda FA
+	and #%11111000
+	bne_16 do_MISSING_FILENAME_error
+
+	lda #$00
+	sta FNLEN
+!:
+	// Fetch device number
+
+	jsr fetch_device_secondary_uint8
+	bcs !+
+
+	sta FA
+!:
+	// Perform loading - just for a different address
+
+	sec
+	lda VARTAB+0
+	sbc #$02
+	sta VARTAB+0
+	tax
+	bcs !+
+	dec VARTAB+1
+!:
+	ldy VARTAB+1
+
+	jmp cmd_load_loadmerge                    // XXX check variable handling (VARTAB) after failure to load