vrtue/src/emu/i8088.rs

use std::fmt::{Debug, Formatter};

use super::byteorder::{ByteOrder, LittleEndian};

use emu::pc::Bus;

#[allow(non_camel_case_types)]
#[derive(Clone, Copy, Debug, Default)]
pub struct i8088 {
  // Data Registers
  pub a: Reg16,
  pub b: Reg16,
  pub c: Reg16,
  pub d: Reg16,

  // Index Registers
  pub si: u16,  // Source Index
  pub di: u16,  // Dest Index
  pub bp: u16,  // Base Pointer
  pub sp: u16,  // Stack Pointer

  // Segment Registers
  pub cs: u16,  // Code Segment
  pub ds: u16,  // Data Segment
  pub es: u16,  // Extra Segment
  pub ss: u16,  // Stack Segment

  // Pointer Register
  pub ip: u16,

  // Status Register
  pub flags: Flags,
}

#[derive(Clone, Copy, Default)]
pub struct Flags {
  cf: bool,  //  0: Carry Flag: 1=CY(Carry), 0=NC(No Carry)
  // 1: Reserved
  pf: bool,  //  2: Parity Flag: 1=PE(Even), 0=PO(Odd)
  // 3: Reserved
  af: bool,  //  4: Adjust Flag: 1=AC(Aux Carry), 0=NA(No Aux Carry)
  // 5: Reserved
  zf: bool,  //  6: Zero Flag: 1=ZR(Zero), 0=NZ(Not Zero)
  sf: bool,  //  7: Sign Flag: 1=NG(Negative), 0=PL(Positive)
  tf: bool,  //  8: Trap Flag
  ie: bool,  //  9: (Real name "IF") Interrupt Enable: 1=EI(Enable Interrupt), 0=DI(Disable Interrupt)
  df: bool,  // 10: Direction Flag: 1=DN(Down), 0=UP(Up)
  of: bool,  // 11: Overflow Flag: 1=OV(Overflow), 0=NV(Not Overflow)
  // bits 12-15 always 1
}

mod ops {
  pub fn call(cpu: &mut super::i8088, mem: &mut [u8], addr: i16) {
    let target = cpu.ip.wrapping_add(addr as u16);
    trace!("CALL 0x{:04X}", target);
    cpu.push16(mem, cpu.ip);
    cpu.ip = target;
  }
}

macro_rules! step {
  (@code $cpu:ident, $bus:ident, ($name:ident, $cycles:literal, $arg:tt)) => {
    step!(@$arg $name $cpu, $bus)
  };

  (@rel16 $name:ident $cpu:ident, $bus:ident) => { {
    let mut buf = [0; 2];
    buf[0] = $cpu.next_ip($bus);
    buf[1] = $cpu.next_ip($bus);
    ops::$name($cpu, &mut $bus.ram, LittleEndian::read_i16(&buf));
  } };

  // Entry Point
  (($cpu:ident, $bus:ident) => { $( $code:literal => $impl:tt ),* } )  => {
    {
      let opcode = $cpu.next_ip($bus);
      match(opcode) {
        $( $code => step!(@code $cpu, $bus, $impl) )*,
        _ => unimplemented!("opcode: {:02X}", opcode)
      }
    }
  }
}

pub const fn segoff_to_addr(segment: u16, offset: u16) -> usize {
  let segaddr = (segment as usize) << 4;
  segaddr + offset as usize
}

impl i8088 {
  pub fn run(&mut self, bus: &mut Bus) {
    loop {
      step!((self, bus) => {
        0xE8 => (call, 23, rel16)
      })
    }
  }

  fn push16(&mut self, mem: &mut [u8], val: u16) {
    // XXX: Not checking for stack faults or anything
    self.sp -= 2;
    LittleEndian::write_u16(&mut mem[segoff_to_addr(self.ss, self.sp)..], val);
  }

  fn next_ip<'a>(&mut self, bus: &Bus) -> u8 {
    let eip = segoff_to_addr(self.cs, self.ip);
    self.ip += 1;

    // We'll assume cpu is always executing in RAM. Also assume the
    // IP doesn't reach the end of the segment (My guess is that it
    // should loop within the segment if it did)
    bus.ram[eip - Bus::RAM_LOCATION]
  }
}


impl From<u16> for Flags {
  fn from(flags: u16) -> Self {
    Self {
      cf: flags & 1       != 0,
      pf: flags & 1 << 2  != 0,
      af: flags & 1 << 4  != 0,
      zf: flags & 1 << 6  != 0,
      sf: flags & 1 << 7  != 0,
      tf: flags & 1 << 8  != 0,
      ie: flags & 1 << 9  != 0,
      df: flags & 1 << 10 != 0,
      of: flags & 1 << 11 != 0,
    }
  }
}

impl From<Flags> for u16 {
  fn from(flags: Flags) -> Self {
    0b1111_0000_0010_1010  // Not sure what all reserved bits should be, but it shouldn't matter
      | (flags.cf as u16)
      | (flags.pf as u16) << 2
      | (flags.af as u16) << 4
      | (flags.zf as u16) << 6
      | (flags.sf as u16) << 7
      | (flags.tf as u16) << 8
      | (flags.ie as u16) << 9
      | (flags.df as u16) << 10
      | (flags.of as u16) << 11
  }
}

impl Debug for Flags {
  fn fmt(&self, fmt: &mut Formatter) -> Result<(), std::fmt::Error> {
    use std::fmt::Write;
    fmt.write_str("[ ")?;
    for flag in [ (self.cf, "CF "),
                  (self.pf, "PF "),
                  (self.af, "AF "),
                  (self.zf, "ZF "),
                  (self.sf, "SF "),
                  (self.tf, "TF "),
                  (self.ie, "IF "),
                  (self.df, "DF "),
                  (self.of, "OF ") ].iter() {
      if flag.0 { fmt.write_str(flag.1)? };
    }
    fmt.write_char(']')?;
    Ok(())
  }
}

#[repr(C)]
#[derive(Clone, Copy)]
pub union Reg16 {
  x: u16,
  lh: [u8; 2],
}

impl Debug for Reg16 {
  fn fmt(&self, fmt: &mut Formatter) -> Result<(), std::fmt::Error> {
    unsafe { self.x.fmt(fmt) }
  }
}

impl Default for Reg16 {
  fn default() -> Self { Self { x: 0u16 } }
}