From ed674f2cdb175c7e49957cf612564c4c00ea3ab7 Mon Sep 17 00:00:00 2001
From: Steven Hugg <hugg@fasterlight.com>
Date: Fri, 26 Jun 2020 16:00:04 -0500
Subject: [PATCH] c64: better detection of PRG start addr.

---
 presets/c64/basicheader.dasm | 10 ++++++++++
 presets/c64/skeleton.dasm    | 17 +++++++++++++++++
 src/machine/c64.ts           | 12 ++++++++----
 3 files changed, 35 insertions(+), 4 deletions(-)
 create mode 100644 presets/c64/basicheader.dasm
 create mode 100644 presets/c64/skeleton.dasm

diff --git a/presets/c64/basicheader.dasm b/presets/c64/basicheader.dasm
new file mode 100644
index 00000000..7baf15c7
--- /dev/null
+++ b/presets/c64/basicheader.dasm
@@ -0,0 +1,10 @@
+
+	processor 6502
+        .org	$07FF
+	.word	$0801        ; two-byte start address
+	.org 	$0801        ; start of BASIC program
+
+        .word BASIC_END, 10  ; Next line and current line number
+        .byte $9e," 2064",0  ; SYS 2064
+BASIC_END:
+	.word 0              ; End of program
diff --git a/presets/c64/skeleton.dasm b/presets/c64/skeleton.dasm
new file mode 100644
index 00000000..1b99a15e
--- /dev/null
+++ b/presets/c64/skeleton.dasm
@@ -0,0 +1,17 @@
+
+ include "cartheader.dasm"
+
+Start
+        ldy #0
+.Loop
+        lda Message,y
+        beq .LoopExit
+	jsr $FFD2	; CHROUT
+        iny
+        bne .Loop
+.LoopExit
+	jmp .		; infinite loop
+Message
+	; PETSCII - http://sta.c64.org/cbm64pet.html
+	byte "HELLO WORLD!"
+	byte 13,0
diff --git a/src/machine/c64.ts b/src/machine/c64.ts
index 274f6d34..f7ba94e1 100644
--- a/src/machine/c64.ts
+++ b/src/machine/c64.ts
@@ -580,8 +580,12 @@ export class C64_WASMMachine extends BaseWASMMachine implements Machine {
     // load rom
     if (this.romptr && this.romlen) {
       this.exports.machine_load_rom(this.sys, this.romptr, this.romlen);
-      this.prgstart = this.romarr[0] + (this.romarr[1]<<8); // TODO: get starting address
-      if (this.prgstart == 0x801) this.prgstart = 0x80d;
+      this.prgstart = this.romarr[0] + (this.romarr[1]<<8); // get load address
+      // look for BASIC program start
+      if (this.prgstart == 0x801) {
+        this.prgstart = this.romarr[2] + (this.romarr[3]<<8) + 2; // point to after BASIC program
+        console.log("prgstart", hex(this.prgstart));
+      }
     }
     // clear keyboard
     for (var ch=0; ch<128; ch++) {
@@ -593,8 +597,8 @@ export class C64_WASMMachine extends BaseWASMMachine implements Machine {
       this.exports.machine_exec(this.sys, 150000);
       // set IRQ routine @ 0x314
       var old0x314 = this.read(0x314) + this.read(0x315) * 256;
-      this.write(0x314, 0x0d);
-      this.write(0x315, 0x08);
+      this.write(0x314, this.prgstart & 0xff);
+      this.write(0x315, (this.prgstart >> 8) & 0xff);
       // wait until IRQ fires
       for (var i=0; i<50000 && this.getPC() != this.prgstart; i++) {
         this.exports.machine_tick(this.sys);