start of rom_only PC

core/src/periph/at28c256/mod.rs

@@ -1,5 +1,6 @@
-mod default;
-mod rom_chip;
+pub mod default;
+pub mod rom_chip;
+pub mod tick;
 
 use crate::constants::constants_system::SIZE_32KB;
 use crate::periph::rom_chip::RomChip;

core/src/periph/at28c256/tick.rs

@@ -0,0 +1,31 @@
+use crate::periph::at28c256::At28C256;
+use crate::periph::hm62256::Hm62256;
+
+impl At28C256 {
+    fn tick(&mut self, address_bus: u16, data_bus: u8, read_mode: bool) -> (u16, u8) {
+        if !read_mode {
+            // has to be read mode.  its a rom.
+            return (address_bus, data_bus)
+        }
+        (address_bus, self.data[address_bus as usize])
+    }
+}
+
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn smoke() { assert!(true); }
+
+    #[test]
+    fn write_to_memory_read_back_works_at_0() {
+        let mut rom = At28C256::default();
+
+        rom.tick(0x0000, 0xab, false);
+        let (_, new_data) = rom.tick(0x0000, 0x00, true);
+
+        assert_eq!(new_data, 0xab);
+    }
+}