From 6ffe28a8a4aae1577c64e2be6eacc42b080b8e00 Mon Sep 17 00:00:00 2001
From: Maxim Poliakovski <maximumspatium@googlemail.com>
Date: Mon, 31 Oct 2022 23:20:39 +0100
Subject: [PATCH] Implement SCSI Pseudo-DMA register on PDM.

---
 devices/common/scsi/sc53c94.cpp | 30 ++++++++++++++++++++++++++++++
 devices/common/scsi/sc53c94.h   |  5 +++--
 devices/ioctrl/amic.cpp         |  6 +++++-
 3 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/devices/common/scsi/sc53c94.cpp b/devices/common/scsi/sc53c94.cpp
index 753c837..1073727 100644
--- a/devices/common/scsi/sc53c94.cpp
+++ b/devices/common/scsi/sc53c94.cpp
@@ -152,6 +152,36 @@ void Sc53C94::write(uint8_t reg_offset, uint8_t value)
     }
 }
 
+uint16_t Sc53C94::pseudo_dma_read()
+{
+    uint16_t data_word;
+    bool     is_done = false;
+
+    if (this->data_fifo_pos >= 2) {
+        // remove one word from FIFO
+        data_word = (this->data_fifo[0] << 8) | this->data_fifo[1];
+        this->data_fifo_pos -= 2;
+        std:memmove(this->data_fifo, &this->data_fifo[2], this->data_fifo_pos);
+
+        // update DMA status
+        if ((this->cmd_fifo[0] & 0x80)) {
+            this->xfer_count -= 2;
+            if (!this->xfer_count) {
+                is_done = true;
+                this->cur_state = SeqState::XFER_END;
+                this->sequencer();
+            }
+        }
+    }
+
+    // see if we need to refill FIFO
+    if (!this->data_fifo_pos && !is_done) {
+        this->sequencer();
+    }
+
+    return data_word;
+}
+
 void Sc53C94::update_command_reg(uint8_t cmd)
 {
     if (this->on_reset && (cmd & 0x7F) != CMD_NOP) {
diff --git a/devices/common/scsi/sc53c94.h b/devices/common/scsi/sc53c94.h
index 332b643..095de73 100644
--- a/devices/common/scsi/sc53c94.h
+++ b/devices/common/scsi/sc53c94.h
@@ -144,8 +144,9 @@ public:
     int device_postinit();
 
     // 53C94 registers access
-    uint8_t read(uint8_t reg_offset);
-    void   write(uint8_t reg_offset, uint8_t value);
+    uint8_t  read(uint8_t reg_offset);
+    void     write(uint8_t reg_offset, uint8_t value);
+    uint16_t pseudo_dma_read();
 
     // ScsiDevice methods
     void notify(ScsiBus* bus_obj, ScsiMsg msg_type, int param);
diff --git a/devices/ioctrl/amic.cpp b/devices/ioctrl/amic.cpp
index e2d234c..7a9b7e9 100644
--- a/devices/ioctrl/amic.cpp
+++ b/devices/ioctrl/amic.cpp
@@ -115,7 +115,11 @@ uint32_t AMIC::read(uint32_t rgn_start, uint32_t offset, int size)
     case 0xA: // MACE registers
         return this->mace->read((offset >> 4) & 0x1F);
     case 0x10: // SCSI registers
-        return this->scsi->read((offset >> 4) & 0xF);
+        if (offset & 0x100) {
+            return this->scsi->pseudo_dma_read();
+        } else {
+            return this->scsi->read((offset >> 4) & 0xF);
+        }
     case 0x14: // Sound registers
         switch (offset) {
         case AMICReg::Snd_Stat_0: