From 1b48fbe6a2ea7ee2a1f1b2b8b0b89b343ef6d8d2 Mon Sep 17 00:00:00 2001
From: Ben Kurtovic <ben.kurtovic@gmail.com>
Date: Tue, 26 Apr 2016 02:19:51 -0500
Subject: [PATCH] Add Z80 INT support and VDP frame interrupts.

---
 src/io.c  | 11 +++++++++++
 src/io.h  |  1 +
 src/vdp.c | 19 ++++++++++++++++++-
 src/vdp.h |  1 +
 src/z80.c | 22 ++++++++++++++++++++++
 5 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/src/io.c b/src/io.c
index fc89e8a..40ee924 100644
--- a/src/io.c
+++ b/src/io.c
@@ -11,12 +11,23 @@ void io_init(IO *io, VDP *vdp)
     io->vdp = vdp;
 }
 
+/*
+    "Power on" the IO object.
+*/
 void io_power(IO *io)
 {
     io->except = false;
 }
 
 /*
+    Return whether the IRQ line is currently active.
+*/
+bool io_check_irq(IO *io)
+{
+    return vdp_assert_irq(io->vdp);
+}
+
+/*
     Read and return a byte from the given port.
 */
 uint8_t io_port_read(IO *io, uint8_t port)
diff --git a/src/io.h b/src/io.h
index 350a5f7..8be1cc1 100644
--- a/src/io.h
+++ b/src/io.h
@@ -20,5 +20,6 @@ typedef struct {
 
 void io_init(IO*, VDP*);
 void io_power(IO*);
+bool io_check_irq(IO*);
 uint8_t io_port_read(IO*, uint8_t);
 void io_port_write(IO*, uint8_t, uint8_t);
diff --git a/src/vdp.c b/src/vdp.c
index 4a89642..eb67aed 100644
--- a/src/vdp.c
+++ b/src/vdp.c
@@ -60,6 +60,14 @@ void vdp_power(VDP *vdp)
 }
 
 /*
+    Return whether frame-completion interrupts are enabled.
+*/
+static bool should_frame_interrupt(const VDP *vdp)
+{
+    return vdp->regs[0x01] & 0x20;
+}
+
+/*
     Return the base address of the pattern name table.
 */
 static uint16_t get_pnt_base(const VDP *vdp)
@@ -115,7 +123,8 @@ void vdp_simulate_line(VDP *vdp)
     if (vdp->v_counter >= 0x18 && vdp->v_counter < 0xA8) {
         // TODO: draw current line
     }
-    // TODO: if (...) IRQ
+    if (vdp->v_counter == 0xC0)
+        vdp->stat_int = true;
     advance_scanline(vdp);
 }
 
@@ -225,6 +234,14 @@ void vdp_write_data(VDP *vdp, uint8_t byte)
 }
 
 /*
+    Return whether the VDP is currently asserting an interrupt.
+*/
+bool vdp_assert_irq(VDP *vdp)
+{
+    return vdp->stat_int && should_frame_interrupt(vdp);
+}
+
+/*
     @DEBUG_LEVEL
     Print out all register values to stdout.
 */
diff --git a/src/vdp.h b/src/vdp.h
index 5d30bb3..46b8d22 100644
--- a/src/vdp.h
+++ b/src/vdp.h
@@ -41,5 +41,6 @@ uint8_t vdp_read_control(VDP*);
 uint8_t vdp_read_data(VDP*);
 void vdp_write_control(VDP*, uint8_t);
 void vdp_write_data(VDP*, uint8_t);
+bool vdp_assert_irq(VDP*);
 
 void vdp_dump_registers(const VDP*);
diff --git a/src/z80.c b/src/z80.c
index 593d1e1..1ad5d89 100644
--- a/src/z80.c
+++ b/src/z80.c
@@ -274,6 +274,24 @@ static inline uint8_t get_interrupt_mode(const Z80 *z80)
 }
 
 /*
+    Handle an active IRQ line. Return the number of cycles consumed.
+*/
+static inline uint8_t handle_interrupt(Z80 *z80)
+{
+    z80->regfile.iff1 = z80->regfile.iff2 = 0;
+    stack_push(z80, z80->regfile.pc);
+
+    if (get_interrupt_mode(z80) < 2) {
+        z80->regfile.pc = 0x0038;
+        return 13;
+    } else {
+        uint16_t addr = (z80->regfile.i << 8) + 0xFF;
+        z80->regfile.pc = mmu_read_double(z80->mmu, addr);
+        return 19;
+    }
+}
+
+/*
     Increment the refresh counter register, R.
 */
 static inline void increment_refresh_counter(Z80 *z80)
@@ -324,6 +342,10 @@ bool z80_do_cycles(Z80 *z80, double cycles)
 {
     cycles += z80->pending_cycles;
     while (cycles > 0 && !z80->except) {
+        if (io_check_irq(z80->io) && z80->regfile.iff1) {
+            cycles -= handle_interrupt(z80);
+            continue;
+        }
         uint8_t opcode = mmu_read_byte(z80->mmu, z80->regfile.pc);
         increment_refresh_counter(z80);
         if (TRACE_LEVEL)