COMBOOT and COM32 files Syslinux supports simple standalone programs, using a file format similar to DOS ".com" files. A 32-bit version, called COM32, is also provided. A simple API provides access to a limited set of filesystem and console functions. ++++ COMBOOT file format ++++ A COMBOOT file is a raw binary file containing 16-bit code. It should be linked to run at offset 0x100, and contain no absolute segment references. It is run in 16-bit real mode. A COMBOOT image can be written to be compatible with MS-DOS. Such a file will usually have extension ".com". A COMBOOT file which is not compatible with MS-DOS will usually have extension ".cbt". Before running the program, Syslinux sets up the following fields in the Program Segment Prefix (PSP), a structure at offset 0 in the program segment: Offset Size Meaning 0 word Contains an INT 20h instruction 2 word Contains the paragraph (16-byte "segment" address) at the end of memory available to the program. 128 byte Length of the command line arguments, including the leading space but not including the final CR character. 129 127b Command line arguments, starting with a space and ending with a CR character (ASCII 13). The program is allowed to use memory between the PSP paragraph (which all the CS, DS, ES and SS registers point to at program start) and the paragraph value given at offset 2. On startup, SP is set up to point to the end of the 64K segment, at 0xfffe. Under DOS it is possible for SP to contain a smaller value if memory is very tight; this is never the case under Syslinux. The program should make no assumptions about what segment address it will be loaded at; instead it should look at the segment registers on program startup. Both DOS and Syslinux will guarantee CS == DS == ES == SS on program start; the program should not assume anything about the values of FS or GS. To exit, a program can either execute a near RET (which will jump to offset 0 which contains an INT 20h instruction, terminating the program), or execute INT 20h or INT 21h AH=00h or INT 21h AH=4Ch. If compatiblity with Syslinux 1.xx is desired, use INT 20h. ++++ COM32 file format ++++ A COM32 file is a raw binary file containing 32-bit code. It should be linked to run at address 0x101000, and should not contain any segment references. It will be run in flat-memory 32-bit protected mode. Under Syslinux, it will be run in CPL 0, however, since it may be possible to create a COM32 execution engine that would run under something like Linux DOSEMU, it is recommended that the code does not assume CPL 0 unless absolutely necessary. It is highly recommended that every COM32 program begins with the byte sequence B8 FF 4C CD 21 (mov eax,21cd4cffh) as a magic number. A COM32 file should have extension ".c32". On startup, CS will be set up as a flat 32-bit code segment, and DS == ES == SS will be set up as the equivalent flat 32-bit data segment. FS and GS are reserved for future use and are currently initialized to zero. A COM32 image should not assume any particular values of segment selectors. ESP is set up at the end of available memory and also serves as notification to the program how much memory is available. The following arguments are passed to the program on the stack: Address Size Meaning [ESP] dword Return (termination) address [ESP+4] dword Number of additional arguments (currently 7) [ESP+8] dword Pointer to the command line arguments (null-terminated string) [ESP+12] dword Pointer to INT call helper function [ESP+16] dword Pointer to low memory bounce buffer [ESP+20] dword Size of low memory bounce buffer [ESP+24] dword Pointer to FAR call helper function (new in 2.05) [ESP+28] dword Pointer to CDECL helper function (new in 3.54) [ESP+32] dword Amount of memory controlled by the Syslinux core (new in 3.74) This corresponds to the following C prototype, available in the file com32/include/com32.h: /* The standard prototype for _start() */ int _start(unsigned int __nargs, char *__cmdline, void (*__intcall)(uint8_t, com32sys_t *, com32sys_t *), void *__bounce_ptr, unsigned int __bounce_len, void (*__farcall)(uint32_t, com32sys_t *, com32sys_t *), int (*__cfarcall)(uint32_t, void *, size_t) ); The intcall helper function can be used to issue BIOS or Syslinux API calls, and takes the interrupt number as first argument. The second argument is a pointer to the input register definition, an instance of the following structure (available in <com32.h>): typedef union { uint32_t l; uint16_t w[2]; uint8_t b[4]; } reg32_t; typedef struct { uint16_t gs; /* Offset 0 */ uint16_t fs; /* Offset 2 */ uint16_t es; /* Offset 4 */ uint16_t ds; /* Offset 6 */ reg32_t edi; /* Offset 8 */ reg32_t esi; /* Offset 12 */ reg32_t ebp; /* Offset 16 */ reg32_t _unused_esp; /* Offset 20 */ reg32_t ebx; /* Offset 24 */ reg32_t edx; /* Offset 28 */ reg32_t ecx; /* Offset 32 */ reg32_t eax; /* Offset 36 */ reg32_t eflags; /* Offset 40 */ } com32sys_t; The third argument is a pointer to the output register definition, an instance of the same structure. The third argument can also be zero (NULL). Since BIOS or Syslinux API calls can generally only manipulate data below address 0x100000, a "bounce buffer" in low memory, at least 64K in size, is available, to copy data in and out. The farcall helper function behaves similarly, but takes as its first argument the CS:IP (in the form (CS << 16) + IP) of procedure to be invoked via a FAR CALL. The cfarcall helper function takes (CS << 16)+IP, a pointer to a stack frame, a size of that stack frame, and returns the return value of EAX (which may need to be appropriate truncated by the user.) ++++ SYSLINUX API CALLS +++ Syslinux provides the following API calls. Syslinux 1.xx only supported INT 20h - terminate program. [] indicates the first version of Syslinux which supported this feature (correctly.) NOTE: Most of the API functionality is still experimental. Expect to find bugs. ++++ DOS-COMPATIBLE API CALLS ++++ INT 20h [1.48] Terminate program INT 21h AH=00h [2.00] Terminate program INT 21h AH=4Ch [2.00] Terminate program All of these terminate the program. INT 21h AH=01h [2.01] Get Key with Echo Reads a key from the console input, with echo to the console output. The read character is returned in AL. Extended characters received from the keyboard are returned as NUL (00h) + the extended character code. INT 21h AH=02h [2.01] Write Character Writes a character in DL to the console (video and serial) output. INT 21h AH=04h [2.01] Write Character to Serial Port Writes a character in DL to the serial console output (if enabled.) If no serial port is configured, this routine does nothing. INT 21h AH=08h [2.09] Get Key without Echo Reads a key fron the console input, without echoing it to the console output. The read character is returned in AL. INT 21h AH=09h [2.01] Write DOS String to Console Writes a DOS $-terminated string in DS:DX to the console. INT 21h AH=0Bh [2.00] Check Keyboard Returns AL=FFh if there is a keystroke waiting (which can then be read with INT 21h, AH=01h or AH=08h), otherwise AL=00h. INT 21h AH=30h [2.00] Check DOS Version This function returns AX=BX=CX=DX=0, corresponding to a hypothetical "DOS 0.0", but the high parts of EAX-EBX-ECX-EDX spell "SYSLINUX": EAX=59530000h EBX=4C530000h ECX=4E490000h EDX=58550000h This function can thus be used to distinguish running on Syslinux from running on DOS. ++++ SYSLINUX-SPECIFIC API CALLS ++++ Syslinux-specific API calls are executed using INT 22h, with a function number in AX. INT 22h is used by DOS for internal purposes; do not execute INT 22h under DOS. DOS-compatible function INT 21h, AH=30h can be used to detect if the Syslinux API calls are available. Any register not specifically listed as modified is preserved; however, future versions of Syslinux may add additional output registers to existing calls. All calls return CF=0 on success, CF=1 on failure. The noted outputs apply if CF=0 only unless otherwise noted. All calls clobber the arithmetric flags (CF, PF, AF, ZF, SF and OF) but leave all other flags unchanged unless otherwise noted. AX=0001h [2.00] Get Version Input: AX 0001h Output: AX number of INT 22h API functions available CH Syslinux major version number CL Syslinux minor version number DL Syslinux derivative ID (e.g. 32h = PXELINUX) ES:SI Syslinux version string ES:DI Syslinux copyright string This API call returns the Syslinux version and API information. AX=0002h [2.01] Write String Input: AX 0002h ES:BX null-terminated string Output: None Writes a null-terminated string on the console. AX=0003h [2.01] Run command Input: AX 0003h ES:BX null-terminated command string Output: Does not return This API call terminates the program and executes the command string as if the user had entered it at the Syslinux command line. This API call does not return. AX=0004h [2.01] Run default command Input: AX 0004h Output: Does not return This API call terminates the program and executes the default command string as if the user had pressed Enter alone on the Syslinux command line. This API call does not return. AX=0005h [2.00] Force text mode Input: AX 0005h Output: None If the screen was in graphics mode (due to displaying a splash screen using the <Ctrl-X> command in a message file, or similar), return to text mode. AX=0006h [2.08] Open file Input: AX 0006h ES:SI null-terminated filename Output: SI file handle EAX length of file in bytes, or -1 CX file block size Open a file for reading. The exact syntax of the filenames...
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