check that block address leaves room for program startup logic

compiler/res/prog8lib/cx16/syslib.p8

@@ -1238,11 +1238,10 @@ sub search_x16edit() -> ubyte {
     str @shared signature = petscii:"x16edit"
     for cx16.r1L in 31 downto 0  {
         cx16.rombank(cx16.r1L)
-        cx16.r2 = $fff0
         %asm {{
             ldy  #0
 -           lda  signature,y
-            cmp  (cx16.r2),y
+            cmp  $fff0,y
             bne  +
             iny
             cpy #7

compiler/src/prog8/compiler/Compiler.kt

@@ -107,14 +107,14 @@ fun compileProgram(args: CompilerArguments): CompilationResult? {
                 )
             }
 
+            determineProgramLoadAddress(program, compilationOptions, args.errors)
+            args.errors.report()
             postprocessAst(program, args.errors, compilationOptions)
+            args.errors.report()
 
 //            println("*********** COMPILER AST BEFORE ASSEMBLYGEN *************")
 //            printProgram(program)
 
-            determineProgramLoadAddress(program, compilationOptions, args.errors)
-            args.errors.report()
-
             if (args.writeAssembly) {
 
                 // re-initialize memory areas with final compilationOptions

compiler/src/prog8/compiler/astprocessing/AstChecker.kt

@@ -280,8 +280,13 @@ internal class AstChecker(private val program: Program,
             errors.err("identifiers cannot start with an underscore", block.position)
 
         val addr = block.address
-        if(addr!=null && addr>65535u) {
-            errors.err("block memory address must be valid integer 0..\$ffff", block.position)
+        if (addr!=null) {
+            if (addr > 65535u)
+                errors.err("block address must be valid integer 0..\$ffff", block.position)
+            if(compilerOptions.loadAddress!=0u) {
+                if (addr < compilerOptions.loadAddress + 20u)
+                    errors.err("block address must be at least program load address + 20 (to allow for startup logic)", block.position)
+            }
         }
 
         for (statement in block.statements) {

compiler/test/codegeneration/TestVariousCodeGen.kt

@@ -384,6 +384,32 @@ main {
         compileText(VMTarget(), false, text, writeAssembly = true) shouldNotBe null
     }
 
+    test("block start address must be greater than program load address") {
+        val src = """
+%output raw
+%launcher none
+%address ${'$'}2000
+
+main $2000 {
+    sub start() {
+        sys.clear_carry()
+    }
+}
+
+otherblock ${'$'}2013 {
+    %option force_output
+}
+
+thirdblock ${'$'}2014 {
+    %option force_output
+}"""
+        val errors = ErrorReporterForTests()
+        compileText(C64Target(), false, src, writeAssembly = false, errors = errors) shouldBe null
+        errors.errors.size shouldBe 2
+        errors.errors[0] shouldContain "6:1: block address must be at least program load address + 20"
+        errors.errors[1] shouldContain "12:1: block address must be at least program load address + 20"
+    }
+
     test("for loops with just 1 iteration") {
         val src="""
 main {

examples/test.p8

@@ -1,50 +1,18 @@
-%zeropage basicsafe
+; EXAMPLE external command source code
+
+%launcher none
 %option no_sysinit
+%zeropage basicsafe
+%encoding iso
+%import textio
+%address $4000
 
-main {
-    sub func() -> ubyte {
-        cx16.r0++
-        return cx16.r0L
-    }
 
-    sub start() {
-        bool[256] @shared cells
-        word starw
-        byte bb
-        uword uw
-        ubyte ub
+main $4030 {
+    %option force_output
 
-        starw = (240-64 as word) + func()
-
-        for starw in 50 downto 10  {
-            cx16.r0++
-        }
-        for starw in cx16.r0L downto 10  {
-            cx16.r0++
-        }
-
-        for ub in 0 to len(cells)-1 {
-            cx16.r0++
-        }
-        for ub in cx16.r0L to len(cells)-1 {
-            cx16.r0++
-        }
-        for bb in 50 downto 10  {
-            cx16.r0++
-        }
-        for bb in cx16.r0sL downto 10  {
-            cx16.r0++
-        }
-
-        for starw in 500 downto 10  {
-            cx16.r0++
-        }
-        for uw in 50 downto 10 {
-            cx16.r0++
-        }
-        for uw in 500 downto 10 {
-            cx16.r0++
-        }
+    sub start()  {
+        txt.print("external command\n")
+        sys.exit(0)
     }
 }
-