variable support cleanup

src/basic/engine_runtime/do_error.s

@@ -0,0 +1,221 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# M65 BASIC_1 #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+// As many errors are triggered from many places in the BASIC
+// interprettor, it is important to have an efficicent means of
+// calling them.
+// LDX #$nn / jmp do_basic_error
+// is the simplest, but takes 5 bytes.
+// Having a direct JMP address for each error is preferable, as
+// it reduces the overhead to 3 bytes, but at the cost of having
+// a jump table of some sort. The .byte $2C method can be used
+// to have a set of addresses that each execute a different
+// LDX #$nn command, reducing the cost of the jump table to 3
+// bytes per error number.
+// But we can do better with a bit of scullduggery and carnal
+// knowledge of ZP usage.
+// The INC $nn opcode is $E6. ZP location $E6 is one of the screen
+// link addresses.  In the original KERNAL, the lower bits of those
+// addresses have meaning, but in our KERNAL, we ignore them, and just
+// have them all zeroes.  So.... We can have a sequence of $E6 bytes,
+// and know exactly how many of them have been executed, by checking
+// the value of $E6.  Then all we need to worry about is if an odd
+// number is required, then we have to have an exit that has a $E6
+// opcode with some other address as target, but where the byte value
+// of that argument can be safely executed as a CPU instruction.
+// NOP = $EA is the obvious choice, as if used as an argument to
+// $E6 will increment location $EA, which is another of the screen
+// link addresses.
+
+// This gives a total overhead of 18 bytes + 1 byte per error
+// message.  This compares favourably to the 3 bytes per error
+// message that the $2C method would result in.
+
+
+	// Error messages reported by KERNAL
+
+do_kernal_error:                       // .A = KERNAL error code, also almost matches BASIC error codes
+
+	// Convert to BASIC error code
+
+	tax
+	beq do_BREAK_error                 // XXX is 0 really a proper error code?
+
+	dex
+#if (ROM_LAYOUT_M65 && SEGMENT_BASIC_1)
+	jmp do_basic_error
+#else
+	bpl do_basic_error                 // branch always
+#endif
+
+	// Error messages specific to OpenROMs BASIC dialect
+
+do_MEMORY_CORRUPT_error:
+	.byte $E6
+
+	// Error messages compatible with CBM BASIC V7 dialect
+
+do_FILE_READ_error:
+	.byte $E6
+do_NOT_IMPLEMENTED_error:
+	.byte $E6
+do_UNRESOLVED_REFERENCE_error:
+	.byte $E6
+do_LINE_NUMBER_TOO_LARGE_error:
+	.byte $E6
+do_BEND_NOT_FOUND_error:
+	.byte $E6
+do_BAD_DISK_error:
+	.byte $E6
+do_NO_GRAPHICS_AREA_error:
+	.byte $E6
+do_DIRECT_MODE_ONLY_error:
+	.byte $E6
+do_LOOP_WITHOUT_DO_error:
+	.byte $E6
+do_LOOP_NOT_FOUND_error:
+	.byte $E6
+do_CANT_RESUME_error:
+	.byte $E6
+
+	// Error messages compatible with CBM BASIC V2 dialect
+
+do_BREAK_error:
+	.byte $E6
+do_LOAD_error:
+	.byte $E6
+do_VERIFY_error:
+	.byte $E6
+do_UNDEFD_FUNCTION_error:
+	.byte $E6
+do_CANT_CONTINUE_error:
+	.byte $E6
+do_FORMULA_TOO_COMPLEX_error:
+	.byte $E6
+do_FILE_DATA_error:
+	.byte $E6
+do_STRING_TOO_LONG_error:
+	.byte $E6
+do_TYPE_MISMATCH_error:
+	.byte $E6
+do_ILLEGAL_DIRECT_error:
+	.byte $E6
+do_DIVISION_BY_ZERO_error:
+	.byte $E6
+do_REDIMD_ARRAY_error:
+	.byte $E6
+do_BAD_SUBSCRIPT_error:
+	.byte $E6
+do_UNDEFD_STATEMENT_error:
+	.byte $E6
+do_OUT_OF_MEMORY_error:
+	.byte $E6
+do_OVERFLOW_error:
+	.byte $E6
+do_ILLEGAL_QUANTITY_error:
+	.byte $E6
+do_OUT_OF_DATA_error:
+	.byte $E6
+do_RETURN_WITHOUT_GOSUB_error:
+	.byte $E6
+do_SYNTAX_error:
+	.byte $E6
+do_NEXT_WITHOUT_FOR_error:
+	.byte $E6
+do_ILLEGAL_DEVICE_NUMBER_error:
+	.byte $E6	
+do_MISSING_FILENAME_error:
+	.byte $E6
+do_NOT_OUTPUT_FILE_error:
+	.byte $E6
+do_NOT_INPUT_FILE_error:
+	.byte $E6
+do_DEVICE_NOT_PRESENT_error:
+	.byte $E6
+do_FILE_NOT_FOUND_error:
+	.byte $E6
+do_FILE_NOT_OPEN_error:
+	.byte $E6
+do_FILE_OPEN_error:
+	.byte $E6
+do_TO_MANY_FILES_error:
+	.byte $EA
+
+#if (ROM_LAYOUT_M65 && SEGMENT_BASIC_1)
+
+	jmp do_error_fetch
+
+#else
+
+	// FALLTROUGH
+
+do_error_fetch:
+
+	// Now get the error id and restore $E6 to its correct value
+	
+	lda $E6
+	pha
+	and #$80
+	sta $E6
+	pla
+	// $E6 has even part
+	asl
+	ora $EA
+	and #$7f
+	tax
+
+	// Restore $EA to correct value in case the NOP above is used
+	// as an argument to the preceeding INC $nn (opcode $E6).
+	
+	lda $EA
+	and #$80
+	sta $EA
+
+	// FALLTHROUGH
+
+do_basic_error:                        // error code in .X
+
+#if ROM_LAYOUT_M65
+	// Make sure we are back with normal memory map
+	jsr map_NORMAL
+#endif
+
+	// "?"
+	
+	phx_trash_a
+	ldx #IDX__STR_RET_QM
+	jsr print_packed_misc_str
+	plx_trash_a
+
+	// Error message text + " ERROR"
+
+	jsr print_packed_error
+
+	ldx #IDX__STR_ERROR
+	jsr print_packed_misc_str
+
+	// Check if direct mode
+
+	lda CURLIN+1
+	cmp #$FF
+	beq !+
+
+	// We were in a program, so show IN <line>
+
+	ldx #IDX__STR_IN
+	jsr print_packed_misc_str
+
+	lda CURLIN+1
+	ldx CURLIN+0
+	jsr print_integer
+!:
+	// Reset stack, and go back to main loop
+
+	ldx #$FE
+	txs
+
+	jmp shell_main_loop
+
+#endif // ROM layout