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 16:16 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.