From d7e918dabe9d52b6eeb76d68b099157625bd37c0 Mon Sep 17 00:00:00 2001
From: Natalia Portillo <claunia@claunia.com>
Date: Wed, 24 Sep 2025 17:50:05 +0100
Subject: [PATCH] Implement READ BUFFER CAPACITY command for CD-ROM

---
 src/cdrom/cdrom.c               | 35 +++++++++++++++++++++++++++++++++
 src/include/86box/cdrom.h       |  1 +
 src/include/86box/scsi_device.h |  1 +
 src/scsi/scsi_cdrom.c           | 22 +++++++++++++++++++++
 4 files changed, 59 insertions(+)

diff --git a/src/cdrom/cdrom.c b/src/cdrom/cdrom.c
index f6c7e9297..7f6c299b0 100644
--- a/src/cdrom/cdrom.c
+++ b/src/cdrom/cdrom.c
@@ -3455,4 +3455,39 @@ int
 cdrom_send_cuesheet(cdrom_t *dev, const uint8_t *buffer, int len)
 {
     return dev->ops->send_cuesheet(dev->local, buffer, len);
+}
+
+int
+cdrom_read_buffer_capacity(cdrom_t *dev, uint8_t *buffer, int block)
+{
+    /* Buffer capacity hardcoded to 1MiB */
+
+    memset(buffer, 0, 12);
+
+    buffer[0] = 0x00; /* Data Length (MSB) */
+    buffer[1] = 0x0a; /* Data Length (LSB) */
+    buffer[2] = 0x00; /* Reserved */
+    buffer[3] = 0x00; /* Reserved */
+    if (block) { /* 512 blocks */
+        buffer[4] = 0x00; /* Length of the Buffer (MSB...) */
+        buffer[5] = 0x00; /* ... */
+        buffer[6] = 0x02; /* ... */
+        buffer[7] = 0x00; /* Length of the Buffer (...LSB) */
+        buffer[8] = 0x00; /* Blank Length of the Buffer (MSB...) */
+        buffer[9] = 0x00; /* ... */
+        buffer[10] = 0x00; /* ... */
+        buffer[11] = 0x00; /* Blank Length of the Buffer (...LSB) */
+    }
+    else { /* 1048576 bytes */
+        buffer[4] = 0x00; /* Length of the Buffer (MSB...) */
+        buffer[5] = 0x10; /* ... */
+        buffer[6] = 0x00; /* ... */
+        buffer[7] = 0x00; /* Length of the Buffer (...LSB) */
+        buffer[8] = 0x00; /* Blank Length of the Buffer (MSB...) */
+        buffer[9] = 0x10; /* ... */
+        buffer[10] = 0x00; /* ... */
+        buffer[11] = 0x00; /* Blank Length of the Buffer (...LSB) */
+    }
+
+    return 12;
 }
\ No newline at end of file
diff --git a/src/include/86box/cdrom.h b/src/include/86box/cdrom.h
index d536470a8..e5c4cca80 100644
--- a/src/include/86box/cdrom.h
+++ b/src/include/86box/cdrom.h
@@ -491,6 +491,7 @@ extern void            cdrom_compute_ecc_block(cdrom_t *dev, uint8_t *parity, co
 extern unsigned long   cdrom_crc32(unsigned long crc, const unsigned char *buf,
                                    size_t len);
 extern int             cdrom_send_cuesheet(cdrom_t *dev, const uint8_t *buffer, int len);
+extern int             cdrom_read_buffer_capacity(cdrom_t *dev, uint8_t *buffer, int block);
 
 extern int             cdrom_assigned_letters;
 
diff --git a/src/include/86box/scsi_device.h b/src/include/86box/scsi_device.h
index a720ca925..203e5af5f 100644
--- a/src/include/86box/scsi_device.h
+++ b/src/include/86box/scsi_device.h
@@ -92,6 +92,7 @@
 #define GPCMD_READ_TRACK_INFORMATION                  0x52
 #define GPCMD_MODE_SELECT_10                          0x55
 #define GPCMD_MODE_SENSE_10                           0x5a
+#define GPCMD_READ_BUFFER_CAPACITY                    0x5c
 #define GPCMD_SEND_CUE_SHEET                          0x5d
 #define GPCMD_PLAY_AUDIO_12                           0xa5
 #define GPCMD_READ_12                                 0xa8
diff --git a/src/scsi/scsi_cdrom.c b/src/scsi/scsi_cdrom.c
index 7e7328fe6..5c090257e 100644
--- a/src/scsi/scsi_cdrom.c
+++ b/src/scsi/scsi_cdrom.c
@@ -98,6 +98,7 @@ uint8_t scsi_cdrom_command_flags[0x100] = {
     [0x51 ... 0x52] = IMPLEMENTED | CHECK_READY,
     [0x55]          = IMPLEMENTED,
     [0x5a]          = IMPLEMENTED,
+    [0x5c]          = IMPLEMENTED | CHECK_READY,
     [0x5d]          = IMPLEMENTED | CHECK_READY,
     [0xa5]          = IMPLEMENTED | CHECK_READY,
     [0xa8 ... 0xa9] = IMPLEMENTED | CHECK_READY,
@@ -3704,6 +3705,27 @@ atapi_out:
             scsi_cdrom_command_complete(dev);
             break;
 
+        case GPCMD_READ_BUFFER_CAPACITY:
+            scsi_cdrom_set_phase(dev, SCSI_PHASE_DATA_IN);
+
+            scsi_cdrom_buf_alloc(dev, 12);
+
+            memset(dev->buffer, 0, 12);
+
+            int block = cdb[1] & 1;
+
+            len = cdrom_read_buffer_capacity(dev->drv, dev->buffer, block);
+
+            if (len < 0) {
+                scsi_cdrom_invalid_field(dev, cdb[1]);
+                break;
+            }
+
+            scsi_cdrom_set_buf_len(dev, BufLen, &len);
+
+            scsi_cdrom_data_command_finish(dev, len, len, len, 0);
+            break;
+
         default:
             scsi_cdrom_illegal_opcode(dev, dev->current_cdb[0]);
             break;