bin2bac: use GOSUB as a much cleaner way to hijack BASICHEADde1

Instead of doing self-modifying BASIC code, use a GOSUB statement
to push the BASIC instruction pointer onto the stack, where it
can be modified if the user so wishes.  See updated bye800.asm
for how to use it.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

3 files changed, 20 insertions, 26 deletions

diff --git a/data/bac800.asm b/data/bac800.asm
index 42c0f1f..7b0190f 100644
--- a/data/bac800.asm
+++ b/data/bac800.asm
@@ -30,8 +30,14 @@ COMSZ:	defw 0			; COMMON area size (memory reserved @ BOTM)
 	defw 0			; Address for the first DEF statement
 	defw 0			; Address for the first DATA statement
 
-	;; BASIC bytecode to invoke _start
+	;; BASIC bytecode to invoke _start The GOSUB allows the
+	;; assembly program to hijack the BASIC flow of control if it
+	;; would want to; the BASIC return instruction pointer is
+	;; found at offset +4 of the value in SPSAV = FF45h
 _basic:
+	defb 130		; GOSUB
+	defw 1+1		; Skip 1 byte
+	defb 146		; END
 	defb 221 + 11		; PUSH int 11
 	defb 209		; Function call, 1 parameter
 	defb 25			; SYS()		SYS(11) = BOFA
@@ -40,10 +46,7 @@ _basic:
 	defb 209		; Function call, 1 parameter
 	defb 23			; CALL()
 	defb 184		; POP int (end expression)
-	defb 128		; GOTO
-gotooffs:
-	defw 1			; Skip nothing
-	defb 146		; END
+	defb 160		; RETURN
 
 	;; Relocator code. In order to support saving and re-loading
 	;; programs, this must handle being restarted at either the
@@ -55,7 +58,6 @@ _start:				; HL points here
 	;; Subtract the length of the (added) relocations from this value
 	;; so that 0 offset = start of program
 	ld bc,_start - _end	; Current base address
-	push hl			; Save pointer to _start
 	ld a,l
 	sub c
 	ld c,a
@@ -113,11 +115,7 @@ reloc_large:
 	; All done, jump to entry point.  A RET will return to BASIC
 	; which will execute an END statement.
 done:
-	pop hl			; -> _start
-	ld de,gotooffs - _start
-	add hl,de		; -> GOTO offset (to override END)
-jpentry:
-	defc REL2=jpentry+1	; Relocation here
+	defc REL2=done+1	; Relocation here
 	jp ENTRY		; Entry relative to start of program
 
 	; Relocation data follows immediately afterwards
diff --git a/data/bin2bac.pl b/data/bin2bac.pl
index 76904b8..ac02303 100755
--- a/data/bin2bac.pl
+++ b/data/bin2bac.pl
@@ -191,16 +191,16 @@ sub makebac800($$$$) {
     my $bhdr;
     # See bac800.asm
     $bhdr  = "\x8f\x00\x01\x00";
-    $bhdr .= pack("v", 0x63 + length($relocs) + length($data));
+    $bhdr .= pack("v", 0x5e + length($relocs) + length($data));
     $bhdr .= pack("v*", 0, 0, 0, $reserve, 0, 0, 0);
-    $bhdr .= "\xe8\xd1\x19\xfd\x9c\xd1\x17\xb8\x80\x01\x00\x92";
-    $bhdr .= "\x01" . pack("v", 0xffc4 - length($relocs));
-    $bhdr .= "\xe5\x7d\x91\x4f\x7c\x98\x47\xb1\x28\x25\xe5\x11\x3a\x00";
+    $bhdr .= "\x82\x02\x00\x92\xe8\xd1\x19\xfe\x9c\xd1\x17\xb8\xa0";
+    $bhdr .= "\x01" . pack("v", 0xffca - length($relocs));
+    $bhdr .= "\x7d\x91\x4f\x7c\x98\x47\xb1\x28\x25\xe5\x11\x34\x00";
     $bhdr .= "\x19\xd1\x7e\xfe\x80\x30\x10\x23\x83\x5f\x30\x01\x14";
     $bhdr .= "\x1a\x81\x12\x13\x1a\x88\x12\x13\x18\xeb\xd6\x81\x38\x06";
-    $bhdr .= "\x23\x82\x57\x7e\x18\xe6\xe1\x11\xfd\xff\x19";
+    $bhdr .= "\x23\x82\x57\x7e\x18\xe6";
     $bhdr .= "\xc3" . pack("v", $entry);
-    $bhdr .= "\x01\x35";
+    $bhdr .= "\x01\x2f";
 
     $data = $bhdr . $relocs . $data;
 
diff --git a/data/bye800.asm b/data/bye800.asm
index 4b7325b..5add19e 100644
--- a/data/bye800.asm
+++ b/data/bye800.asm
@@ -3,18 +3,14 @@
 ;;
 
 	defc CONWRITE=000Bh
-	defc IPSAV=65338
+	defc SPSAV=0FF45h
 	
 _start:
 	;; After this, do CHAIN "NUL:"
-	ld de,chain_nul-1
-	ld a,e
-	sub l
-	ld (hl),a
-	ld a,d
-	sbc h
-	inc hl
-	ld (hl),a
+	ld ix,(SPSAV)
+	ld de,chain_nul
+	ld (ix+4),e
+	ld (ix+5),d
 	
 	ld hl,hello
 	ld c,hello_len