IDE Driver (ide.c / ide.h)

Overview

ide.c implements a minimal ATA PIO (Programmed I/O) driver for the Makar kernel. It supports 28-bit LBA sector reads and writes over two ATA channels (primary and secondary), giving access to up to four drives (primary master, primary slave, secondary master, secondary slave).

Bus-master (BMIDE) DMA is used when a drive advertises it, with a PIO fallback on any failure; completion is detected by polling (no IRQ-driven I/O). ATAPI (CD-ROM) devices are detected and read via the SCSI PACKET command set (READ(12), READ CAPACITY(10), START/STOP UNIT for eject); ATAPI writing is not supported.

Concurrency

The single controller / shared bus-master bounce buffer means only one transfer may be in flight at a time. ide_read_sectors / ide_write_sectors / ide_read_atapi_sectors therefore take an irq-guarded controller lock (ide_lock/ide_unlock): the test-and-set briefly masks interrupts so it is correct from both interrupts-off syscalls and interrupts-on (preemptible) kernel tasks; a task that finds the controller busy yields until it frees. io_spin_pump() animates the boot/status spinner during a wait but does not yield — responsiveness during a long disk operation comes from the timer preempting the (interrupts-enabled) caller, e.g. the installer, rather than cooperative yields inside the poll loop.

Hardware Registers

Each ATA channel exposes two I/O port ranges:

Channel Command block base Control block base
Primary 0x1F0 0x3F6
Secondary 0x170 0x376

Command-block register offsets:

Offset Read Write
+0 Data (16-bit) Data
+1 Error Features
+2 Sector Count -
+3 LBA bits 0–7 -
+4 LBA bits 8–15 -
+5 LBA bits 16–23 -
+6 Drive/Head -
+7 Status Command

The control block base (+0) holds the alternate-status register (read) or the device-control register (write, used to disable IRQs via bit 1 nIEN).

Drive Indices 

Index 0 – primary   master
Index 1 – primary   slave
Index 2 – secondary master
Index 3 – secondary slave

Public API

void ide_init(void)

Scans both channels, runs IDENTIFY DEVICE (or IDENTIFY PACKET DEVICE for ATAPI) on every slot, and populates the internal drive table.

Must be called once during kernel initialisation, after interrupts are disabled for the ATA channels (which this function does itself via the control-block nIEN bit).

Before probing, ide_init issues a software reset (SRST, ATA spec §9.1) on both channels: assert bit 2 of the Device Control register for ≥5 µs, then deassert and wait ~100 µs for drives to recalibrate. This is required because GRUB leaves the primary channel in a transient state after loading the kernel; without the reset, the first IDENTIFY status read returns 0x00 and drive 0 is silently skipped.

int ide_read_sectors(uint8_t drive_num, uint32_t lba, uint8_t count, void *buf)

Reads count 512-byte sectors starting at 28-bit LBA address lba from drive drive_num into buf.

Return value Meaning
0 Success
-1 Drive index out of range or drive not present
-2 Drive is ATAPI - not supported by PIO read
1 ATA error bit set during transfer
2 Drive fault during transfer
3 DRQ not asserted after command

int ide_write_sectors(uint8_t drive_num, uint32_t lba, uint8_t count, const void *buf)

Writes count 512-byte sectors from buf to drive drive_num starting at LBA lba. A cache-flush command is issued automatically after the last sector.

Return values are identical to ide_read_sectors.

ATAPI (CD-ROM) 

int ide_read_atapi_sectors(uint8_t drive_num, uint32_t lba, uint16_t count, void *buf);
int ide_atapi_capacity(uint8_t drive_num, uint32_t *out_sectors, uint32_t *out_sec_size);
int ide_eject_atapi(uint8_t drive_num);
  • ide_read_atapi_sectors reads count 2048-byte sectors via a READ(12) command packet.
  • ide_atapi_capacity issues READ CAPACITY(10) (opcode 0x25) and reports the medium’s sector count (last_lba + 1) and block size (usually 2048). ATAPI IDENTIFY carries no usable LBA range, so devfs uses this to size /dev/cdrom.
  • ide_eject_atapi sends START/STOP UNIT with the eject bit (opens the tray).

All return 0 on success, -1 invalid/absent drive, -2 not ATAPI, positive on a protocol error.

const ide_drive_t *ide_get_drive(uint8_t drive_num)

Returns a pointer to the drive descriptor for drive_num (0–3), or NULL if the index is out of range. Callers must check drive->present before using any other field.

ide_drive_t Structure 

typedef struct {
    uint8_t  present;       /* 1 if a drive exists at this index          */
    uint8_t  channel;       /* 0 = primary, 1 = secondary                 */
    uint8_t  drive;         /* 0 = master,  1 = slave                     */
    uint8_t  type;          /* IDE_TYPE_ATA or IDE_TYPE_ATAPI             */
    uint16_t signature;     /* Device type word from IDENTIFY             */
    uint16_t capabilities;  /* Capabilities word from IDENTIFY            */
    uint32_t command_sets;  /* Supported command sets from IDENTIFY       */
    uint32_t size;          /* Size in 512-byte sectors                   */
    char     model[41];     /* Model string (NUL-terminated, trimmed)     */
} ide_drive_t;

Shell Commands

Two shell commands expose the driver to the interactive kernel shell:

lsdisks

Lists all detected ATA/ATAPI drives with their type, size (in MiB), and model string. Example output:

drive 0: [primary master] ATA  20480 MiB  "QEMU HARDDISK"

readsector <drive> <lba>

Reads one 512-byte sector from the given drive at the given LBA address and prints a hex dump to the terminal. Both arguments can be decimal or 0x-prefixed hexadecimal.

Example:

untitled> readsector 0 0
Sector 0 of drive 0:
0000:  EB 63 90 00 ...

Limitations

  • Only 28-bit LBA is used for read/write. Drives larger than 128 GiB can be detected (their 64-bit sector count is truncated to 32 bits) but only the first 128 GiB is accessible.
  • No DMA support. PIO is slower than DMA for large transfers.
  • No IRQ-driven transfers; the driver busy-waits on status bits.
  • ATAPI is read-only (READ(12)); no ATAPI writing/burning.