source: bootcd/isolinux/syslinux-6.03/com32/gfxboot/realmode_callback.asm

                bits 16

                section .text

                ; must be filled in
f_buf_size      dw 0
f_buf_seg       dw 0


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
gfx_cb:
                push cs
                pop ds

                cmp al,cb_len
                jae gfx_cb_80

                movzx bx,al
                add bx,bx
                call word [bx+cb_table]
                jmp gfx_cb_90

gfx_cb_80:
                mov al,0ffh
gfx_cb_90:
                retf


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
; Return status info.
;
; return:
;  edx          filename buffer (64 bytes)
;
cb_status:
                mov edx,cs
                shl edx,4
                add edx,f_name
                xor al,al
                ret


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
; Open file.
;
; return:
;    al         0: ok, 1: file not found
;   ecx         file length (al = 0)
;
cb_fopen:
                mov si,f_name
                push ds
                pop es
                mov ax,6
                int 22h
                xchg edx,eax
                mov al,1
                jc cb_fopen_90
                cmp cx,[f_buf_size]
                ja cb_fopen_90
                or cx,cx
                jz cb_fopen_90
                mov [f_block_size],cx
                or edx,edx
                jz cb_fopen_90
                mov [f_handle],si
                mov [f_size],edx
                mov ecx,edx
                mov ax,[f_buf_size]
                cwd
                div word [f_block_size]
                mov [f_blocks],ax

                xor al,al
cb_fopen_90:
                ret


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
; Read next chunk.
;
; return:
;   edx         buffer address (linear)
;   ecx         data length (< 64k)
;
cb_fread:
                xor ecx,ecx
                mov si,[f_handle]
                or si,si
                jz cb_fread_80
                mov cx,[f_blocks]
                mov es,[f_buf_seg]
                xor bx,bx
                mov ax,7
                int 22h
                mov [f_handle],si
                mov al,1
                jc cb_fread_90
cb_fread_80:
                xor al,al
cb_fread_90:
                movzx edx,word [f_buf_seg]
                shl edx,4
                ret


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
; Return current working directory.
;
; return:
;  edx          filename
;
cb_getcwd:
                mov ax,1fh
                int 22h
                mov edx,es
                shl edx,4
                movzx ebx,bx
                add edx,ebx
                xor al,al
                ret


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
; Set current working directory.
;
cb_chdir:
                mov bx,f_name
                push ds
                pop es
                mov ax,25h
                int 22h
                xor al,al
                ret


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
; read sector
;
;  edx          sector
;
; return:
;  edx          buffer (linear address)
;
;  Note: does not return on error!
;
cb_readsector:
                xor edi,edi
                xor esi,esi
                mov cx,1
                mov es,[f_buf_seg]
                xor bx,bx
                mov ax,19h
                int 22h
                movzx edx,word [f_buf_seg]
                shl edx,4
                xor al,al
                ret


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
; Re-read fs structures.
;
cb_mount:
                mov ax,26h
                int 22h
                setc al
                ret


; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
;
                align 2, db 0

cb_table        dw cb_status
                dw cb_fopen
                dw cb_fread
                dw cb_getcwd
                dw cb_chdir
                dw cb_readsector
                dw cb_mount
cb_len          equ ($-cb_table)/2

f_handle        dw 0
f_block_size    dw 0
f_blocks        dw 0
f_size          dd 0
f_name          times 64 db 0
f_name_len      equ $ - f_name