diff --git a/codeGenCpu6502/src/prog8/codegen/cpu6502/BuiltinFunctionsAsmGen.kt b/codeGenCpu6502/src/prog8/codegen/cpu6502/BuiltinFunctionsAsmGen.kt
index c2aeda7e5..d24eb4e5d 100644
--- a/codeGenCpu6502/src/prog8/codegen/cpu6502/BuiltinFunctionsAsmGen.kt
+++ b/codeGenCpu6502/src/prog8/codegen/cpu6502/BuiltinFunctionsAsmGen.kt
@@ -612,7 +612,51 @@ internal class BuiltinFunctionsAsmGen(private val program: PtProgram,
     }
 
     private fun funcSetLsbMsb(fcall: PtBuiltinFunctionCall, msb: Boolean) {
-        TODO("setlsb/setmsb for $fcall")
+        asmgen.assignExpressionToRegister(fcall.args[1], RegisterOrPair.A, false)
+
+        val address: PtExpression
+        when(fcall.args[0]) {
+            is PtIdentifier -> {
+                if(msb) {
+                    address = PtBinaryExpression("+", DataType.UWORD, fcall.args[0].position)
+                    val addressOf = PtAddressOf(fcall.position)
+                    addressOf.add(fcall.args[0])
+                    address.add(addressOf)
+                    address.add(PtNumber(address.type, 1.0, fcall.args[0].position))
+                } else {
+                    address = PtAddressOf(fcall.position)
+                    address.add(fcall.args[0])
+                }
+            }
+            is PtNumber -> {
+                val num = (fcall.args[0] as PtNumber).number + if(msb) 1 else 0
+                address = PtNumber(fcall.args[0].type, num, fcall.args[0].position)
+            }
+            is PtAddressOf -> {
+                if(msb) {
+                    address = PtBinaryExpression("+", DataType.UWORD, fcall.args[0].position)
+                    address.add(fcall.args[0])
+                    address.add(PtNumber(address.type, 1.0, fcall.args[0].position))
+                } else {
+                    address = fcall.args[0]
+                }
+            }
+            is PtArrayIndexer -> {
+                val indexer = fcall.args[0] as PtArrayIndexer
+                require(!indexer.usesPointerVariable)
+                if(indexer.splitWords) {
+                    // lsb/msb in split arrays, element index 'size' is always 1
+                    TODO("setlsb/setmsb on split array element ${fcall.position}")
+                } else {
+                    TODO("setlsb/setmsb on array element ${fcall.position}")
+                }
+            }
+            else -> throw AssemblyError("setlsb/setmsb on weird target ${fcall.args[0]}")
+        }
+        val mem = PtMemoryByte(fcall.position)
+        mem.add(address)
+        mem.parent = fcall
+        assignAsmGen.storeRegisterAInMemoryAddress(mem)     // TODO use assignRegisterByte()???,  and assignConstantByte() if the value is contstant zero
     }
 
     private fun funcSgn(fcall: PtBuiltinFunctionCall, resultRegister: RegisterOrPair?, scope: IPtSubroutine?) {
diff --git a/codeGenCpu6502/src/prog8/codegen/cpu6502/assignment/AssignmentAsmGen.kt b/codeGenCpu6502/src/prog8/codegen/cpu6502/assignment/AssignmentAsmGen.kt
index d73f158c1..5868a817a 100644
--- a/codeGenCpu6502/src/prog8/codegen/cpu6502/assignment/AssignmentAsmGen.kt
+++ b/codeGenCpu6502/src/prog8/codegen/cpu6502/assignment/AssignmentAsmGen.kt
@@ -3589,7 +3589,7 @@ internal class AssignmentAsmGen(private val program: PtProgram,
         }
     }
 
-    private fun storeRegisterAInMemoryAddress(memoryAddress: PtMemoryByte) {
+    internal fun storeRegisterAInMemoryAddress(memoryAddress: PtMemoryByte) {
         val addressExpr = memoryAddress.address
         val addressLv = addressExpr as? PtNumber
         val addressOf = addressExpr as? PtAddressOf
diff --git a/codeGenIntermediate/src/prog8/codegen/intermediate/BuiltinFuncGen.kt b/codeGenIntermediate/src/prog8/codegen/intermediate/BuiltinFuncGen.kt
index 6e19ce88f..e974b9848 100644
--- a/codeGenIntermediate/src/prog8/codegen/intermediate/BuiltinFuncGen.kt
+++ b/codeGenIntermediate/src/prog8/codegen/intermediate/BuiltinFuncGen.kt
@@ -597,6 +597,7 @@ internal class BuiltinFuncGen(private val codeGen: IRCodeGen, private val exprGe
             is PtArrayIndexer -> {
                 require(!target.usesPointerVariable)
                 if(target.splitWords) {
+                    // lsb/msb in split arrays, element index 'size' is always 1
                     val varName = target.variable.name + if(msb) "_msb" else "_lsb"
                     val valueTr = exprGen.translateExpression(call.args[1])
                     addToResult(result, valueTr, valueTr.resultReg, -1)
diff --git a/examples/test.p8 b/examples/test.p8
index 682ead226..e3740c8c3 100644
--- a/examples/test.p8
+++ b/examples/test.p8
@@ -7,9 +7,11 @@ main {
         txt.print_uwhex(zz, true)
         txt.nl()
 
+        ;@(&zz) = $11
         setlsb(zz, $11)
         txt.print_uwhex(zz, true)
         txt.nl()
+        ;@(&zz+1) = $22
         setmsb(zz, $22)
         txt.print_uwhex(zz, true)
         txt.nl()
@@ -23,6 +25,8 @@ main {
         txt.nl()
         txt.print_uwhex(array[2], true)
         txt.nl()
+        ;@(&array+one*2) = $ff
+        ;@(&array+two*2+1) = $ff
         setlsb(array[one],$ff)
         setmsb(array[two],$00)
         txt.print_uwhex(array[1], true)