Accidentally had the opcode for TXA be NOP, don't know how that happened. I ported background.asm from NESASM format to my format, it is veeeery similar so not much needed changing, and the resulting ROM worked fine :)

.gitignore

@@ -3,3 +3,4 @@
 Desktop.ini
 Thumbs.db
 *.deb
+*.swp

data/opcodes.yaml

@@ -710,7 +710,7 @@
     - :s
     - :z
   :implied:
-    :hex: 0xEA
+    :hex: 0xE8
     :len: 1
     :cycles: 2
     :boundry_add: false

lib/instruction.rb

@@ -153,7 +153,7 @@ module Assembler6502
       end
 
       ##  We just don't recognize this line of asm, it must be a Syntax Error
-      fail(SyntaxError, sprintf("%.4X: #{asm_line}", address))
+      fail(SyntaxError, sprintf("%.4X: ", address) +  asm_line)
     end
 
 

my_background.asm

@@ -0,0 +1,55 @@
+  ;  Create an iNES header
+  .ines {"prog": 1, "char": 1, "mapper": 0, "mirror": 1}
+
+
+  ;  Main code segment
+  .org $C000 
+RESET:
+  SEI          ; disable IRQs
+  CLD          ; disable decimal mode
+  LDX #$40
+  STX $4017    ; disable APU frame IRQ
+  LDX #$FF
+  TXS          ; Set up stack
+  INX          ; now X = 0
+  STX $2000    ; disable NMI
+  STX $2001    ; disable rendering
+  STX $4010    ; disable DMC IRQs
+
+vblankwait1:       ; First wait for vblank to make sure PPU is ready
+  BIT $2002
+  BPL vblankwait1
+
+clrmem:
+  LDA #$00
+  STA $0000, X
+  STA $0100, X
+  STA $0200, X
+  STA $0400, X
+  STA $0500, X
+  STA $0600, X
+  STA $0700, X
+  LDA #$FE
+  STA $0300, X
+  INX
+  BNE clrmem
+
+vblankwait2:      ; Second wait for vblank, PPU is ready after this
+  BIT $2002
+  BPL vblankwait2
+
+
+  LDA #$60        ;intensify blues
+  STA $2001
+
+Forever:
+  JMP Forever     ;jump back to Forever, infinite loop
+
+NMI:
+  RTI
+
+
+  .org $FFFA     ;first of the three vectors starts here
+  .dw NMI        ;when an NMI happens (once per frame if enabled) the processor will jump to the label NMI:
+  .dw RESET      ;when the processor first turns on or is reset, it will jump to the label RESET:
+  .dw $0         ;external interrupt IRQ is not used in this tutorial