From a6e866c0e547f4bffca287e425bdf598e7966a62 Mon Sep 17 00:00:00 2001
From: Akamaki <97360908+akmed772@users.noreply.github.com>
Date: Tue, 18 Feb 2025 22:34:27 +0900
Subject: [PATCH] added comments

---
 src/disk/hdc_esdi_mca.c | 16 ++++++++++++++--
 src/floppy/fdc.c        |  2 +-
 src/machine/m_ps2_mca.c |  6 +++++-
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/src/disk/hdc_esdi_mca.c b/src/disk/hdc_esdi_mca.c
index 690f658a0..8f1def137 100644
--- a/src/disk/hdc_esdi_mca.c
+++ b/src/disk/hdc_esdi_mca.c
@@ -1471,16 +1471,28 @@ static device_config_t
           { .type = -1 }
 };
 
+/*
+Device for an IBM DBA (Direct Bus Attachment) hard disk.
+The Disk BIOS is included in the System ROM.
+Some models have an exclusive channel slot for the DBA hard disk.
+Following IBM machines are supported:
+  * PS/2 model 55SX
+  * PS/2 model 65SX
+  * PS/2 model 70 type 3 (Slot #4)
+  * PS/2 model 70 type 4 (Slot #4)
+  * PS/55 model 5550-T (Slot #5)
+  * PS/55 model 5550-V (Slot #5)
+*/
 const device_t
 esdi_integrated_device = {
-    .name = "IBM Integrated Fixed Disk Controller (MCA)",
+    .name = "IBM Integrated Fixed Disk and Controller (MCA)",
     .internal_name = "esdi_integrated_mca",
     .flags = DEVICE_MCA,
     .local = ESDI_IS_INTEGRATED,
     .init = esdi_init,
     .close = esdi_close,
     .reset = esdi_reset,
-    .available = NULL, /* The Disk BIOS is included in the System ROM */
+    .available = NULL,
     .speed_changed = NULL,
     .force_redraw = NULL,
     .config = esdi_integrated_config
diff --git a/src/floppy/fdc.c b/src/floppy/fdc.c
index 8a1a33f2f..5de1a8ef4 100644
--- a/src/floppy/fdc.c
+++ b/src/floppy/fdc.c
@@ -740,7 +740,7 @@ fdc_write(uint16_t addr, uint8_t val, void *priv)
                 drive = real_drive(fdc, fdc->dor & 3);
                 fdc_update_rwc(fdc, drive, (val & 0x30) >> 4);
             }
-            /* Bit 2: FIFO test mode (PS/55 only? This is not documented in neither the PS/2 HITR nor the 82077AA datasheet.) 
+            /* Bit 2: FIFO test mode (PS/55 5550-S,T only. Undocumented) 
                The Power-on Self Test of PS/55 writes and verifies 8 bytes of FIFO buffer through I/O 3F5h.
                If it fails, then floppy drives will be treated as DD drives. */
             if (fdc->flags & FDC_FLAG_PS2_MCA) {
diff --git a/src/machine/m_ps2_mca.c b/src/machine/m_ps2_mca.c
index 1edff2991..ac22af2fe 100644
--- a/src/machine/m_ps2_mca.c
+++ b/src/machine/m_ps2_mca.c
@@ -1890,7 +1890,11 @@ machine_ps55_model_50t_init(const machine_t* model)
     * Planar ID
     * FFFAh - PS/55 model 5551-S0x, T0x (stage 1?)
     * FFEEh - PS/55 model 5551-S1x, T1x (stage 2?)
-    * POST (P/N 38F6933) determination: FBxx -> 4 slots (error), xxEE -> 5 slots (ok), others -> 8 (error)
+    * Verification in BIOS P/N 38F6933: FBxx -> 4 slots (error), xxEE -> 5 slots (ok), others -> 8 (error)
+    * 
+    * The only difference between S and T models is the CPU speed (16 MHz vs 20 MHz).
+    * The POST measures the speed, and sets a flag in the BIOS Data Area to indicate the sub model.
+    * The VM in 86Box runs faster than the real, so the POST always determines it as the T model.
     */
     ps2.planar_id = 0xffee;
     ps55_mca_board_model_50t_init();