From d9b4e1fc1f428d5d1ada46410f28ea7d69599fce Mon Sep 17 00:00:00 2001
From: Uwe Seimet <48174652+uweseimet@users.noreply.github.com>
Date: Sat, 5 Feb 2022 02:45:10 +0100
Subject: [PATCH] Support ICD (Atari ACSI) command semantics (#640)

* Initial ICD support

* Comment update

* Comment update

* Comment update

* Update README.md
---
 README.md                   |  2 +-
 src/raspberrypi/gpiobus.cpp | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 5676767f..5c7eb81a 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,5 @@
 # What is RaSCSI?
-RaSCSI is a virtual SCSI device emulator that runs on a Raspberry Pi. It runs in userspace, and can emulate several SCSI devices at one time. There is a control interface to attach / detach drives during runtime, as well as insert and eject removable media. This project is aimed at users of vintage Macintosh computers and more (see [compatibility list](https://github.com/akuker/RASCSI/wiki/Compatibility)) from the 1980's and 1990's.
+RaSCSI is a virtual SCSI device emulator that runs on a Raspberry Pi. It runs in userspace, and can emulate several SCSI devices at one time. There is a control interface to attach / detach drives during runtime, as well as insert and eject removable media. This project is aimed at users of vintage Macintosh and Atari computers and more (see [compatibility list](https://github.com/akuker/RASCSI/wiki/Compatibility)) from the 1980's and 1990's.
 
 Please check out the full story with much more detail on the [wiki](https://github.com/akuker/RASCSI/wiki)!
 
diff --git a/src/raspberrypi/gpiobus.cpp b/src/raspberrypi/gpiobus.cpp
index 10a6f985..f09372af 100644
--- a/src/raspberrypi/gpiobus.cpp
+++ b/src/raspberrypi/gpiobus.cpp
@@ -861,6 +861,38 @@ int GPIOBUS::CommandHandShake(BYTE *buf)
 		goto irq_enable_exit;
 	}
 
+	// The ICD AdSCSI ST, AdSCSI Plus ST and AdSCSI Micro ST host adapters allow SCSI devices to be connected
+	// to the ACSI bus of Atari ST/TT computers and some clones. ICD-aware drivers prepend a $1F byte in front
+	// of the CDB (effectively resulting in a custom SCSI command) in order to get access to the full SCSI
+	// command set. Native ACSI is limited to the low SCSI command classes with command bytes < $20.
+	// Most other host adapters (e.g. LINK96/97 and the one by Inventronik) and also several devices (e.g.
+	// UltraSatan or GigaFile) that can directly be connected to the Atari's ACSI port also support ICD
+	// semantics. I fact, these semantics have become a standard in the Atari world.
+
+	// RaSCSI becomes ICD compatible by ignoring the prepended $1F byte before processing the CDB.
+	if (*buf == 0x1F) {
+		SetSignal(PIN_REQ, ON);
+
+		ret = WaitSignal(PIN_ACK, TRUE);
+
+		SysTimer::SleepNsec(SCSI_DELAY_BUS_SETTLE_DELAY_NS);
+
+		// Get the actual SCSI command
+		*buf = GetDAT();
+
+		SetSignal(PIN_REQ, OFF);
+
+		if (!ret) {
+			goto irq_enable_exit;
+		}
+
+		WaitSignal(PIN_ACK, FALSE);
+
+		if (!ret) {
+			goto irq_enable_exit;
+		}
+	}
+
 	count = GetCommandByteCount(*buf);
 
 	// Increment buffer pointer