When initially transferring control to OS2LDR from a "black box", the following interface is defined:

DH

boot mode flags:

bit 0 (NOVOLIO) on indicates that the mini-FSD does not use MFSH_DOVOLIO.
bit 1 (RIPL) on indicates that boot volume is not local (RIPL boot)
bit 2 (MINIFSD) on indicates that a mini-FSD is present.
bit 3 (RESERVED)
bit 4 (MICROFSD) on indicates that a micro-FSD is present.
bits 5-7 are reserved and MUST be zero.

DL
drive number for the boot disk. This parameter is ignored if either the NOVOLIO or MINIFSD bits are zero.
DS:SI
is a pointer to the BOOT Media's BPB. This parameter is ignored if either the NOVOLIO or MINIFSD bits are zero.
ES:DI
is a pointer to a filetable structure. The filetable structure has the following format:

struct FileTable {
    unsigned short ft_cfiles; /* # of entries in this table             */
    unsigned short ft_ldrseg; /* paragraph # where OS2LDR is loaded     */
    unsigned long  ft_ldrlen; /* length of OS2LDR in bytes              */
    unsigned short ft_museg;  /* paragraph # where microFSD is loaded   */
    unsigned long  ft_mulen;  /* length of microFSD in bytes            */
    unsigned short ft_mfsseg; /* paragraph # where miniFSD is loaded    */
    unsigned long  ft_mfslen; /* length of miniFSD in bytes             */
    unsigned short ft_ripseg; /* paragraph # where RIPL data is loaded  */
    unsigned long  ft_riplen; /* length of RIPL data in bytes           */

    /* The next four elements are pointers to microFSD entry points     */

    unsigned short (far *ft_muOpen)
                     (char far *pName, unsigned long far *pulFileSize);
    unsigned long (far *ft_muRead)
                     (long loffseek, char far *pBuf, unsigned long cbBuf);
    unsigned long (far *ft_muClose)(void);
    unsigned long (far *ft_muTerminate)(void);
}

The microFSD entry points interface is defined as follows:

mu_Open - is passed a far pointer to name of file to be opened and a far pointer to a ULONG to return the file's size. The returned value (in AX) indicates success(0) or failure(non-0).

mu_Read - is passed a seek offset, a far pointer to a data buffer, and the size of the data buffer. The returned value(in DX:AX) indicates the number of bytes actually read.

mu_Close - has no parameters and expects no return value. It is a signal to the micro-FSD that the loader is done reading the current file.

mu_Terminate - has no parameters and expects no return value. It is a signal to the micro-FSD that the loader has finished reading the boot drive.

The loader will call the micro-FSD in a Open-Read-Read-....-Read-Close sequence with each file read in from the boot drive.

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