.code16 .intel_syntax noprefix .section .text .global _start .equ PSP_COMMAND_LENGTH, 0x80 .equ PSP_COMMAND_TAIL, 0x81 .equ SCRATCH_BYTES, 260 /* * RIALIAS.COM - session-scoped DOS pathname drive aliases. * * Mapping data is supplied on the command line. For example, /A=C /B=D maps * explicit A: paths to C: and explicit B: paths to D:. There are no built-in * source or target drives. * * The INT 21h hook copies mapped paths into resident scratch storage. It never * edits the caller's pathname buffer. Unsupported APIs are chained unchanged. */ _start: push cs pop ds xor bp, bp xor ch, ch mov cl, byte ptr [PSP_COMMAND_LENGTH] mov si, PSP_COMMAND_TAIL parse_next: jcxz parse_done mov al, byte ptr [si] cmp al, ' ' je consume_space cmp al, 9 je consume_space cmp al, '/' jne parse_source inc si dec cx jcxz parse_error parse_source: cmp cx, 3 jb parse_error mov al, byte ptr [si] and al, 0xdf cmp al, 'A' jb parse_error cmp al, 'Z' ja parse_error sub al, 'A' xor bx, bx mov bl, al inc si dec cx cmp byte ptr [si], '=' jne parse_error inc si dec cx mov al, byte ptr [si] and al, 0xdf cmp al, 'A' jb parse_error cmp al, 'Z' ja parse_error mov byte ptr cs:[drive_map + bx], al inc bp inc si dec cx jcxz parse_done mov al, byte ptr [si] cmp al, ' ' je consume_space cmp al, 9 jne parse_error consume_space: inc si dec cx jmp parse_next parse_done: test bp, bp jz parse_error mov ax, 0x3521 int 0x21 mov word ptr cs:[old_int21], bx mov word ptr cs:[old_int21 + 2], es push cs pop ds mov dx, offset int21_handler mov ax, 0x2521 int 0x21 mov dx, offset resident_end add dx, 15 shr dx, 4 mov ax, 0x3100 int 0x21 parse_error: mov ax, 0x4c02 int 0x21 int21_handler: pushf cmp byte ptr cs:[busy], 0 jne chain_unchanged cmp ah, 0x39 /* mkdir DS:DX */ je inspect_dx_path cmp ah, 0x3a /* rmdir DS:DX */ je inspect_dx_path cmp ah, 0x3b /* chdir DS:DX */ je inspect_dx_path cmp ah, 0x3c /* create/truncate DS:DX */ je inspect_dx_path cmp ah, 0x3d /* open DS:DX */ je inspect_dx_path cmp ah, 0x41 /* unlink DS:DX */ je inspect_dx_path cmp ah, 0x43 /* get/set attributes DS:DX */ je inspect_dx_path cmp ah, 0x4b /* exec DS:DX */ je inspect_dx_path cmp ah, 0x4e /* find first DS:DX */ je inspect_dx_path cmp ah, 0x56 /* rename DS:DX to ES:DI */ je inspect_rename_paths cmp ah, 0x5b /* create new DS:DX */ je inspect_dx_path cmp ah, 0x60 /* truename DS:SI to ES:DI */ je inspect_si_path cmp ah, 0x6c /* extended open DS:SI */ je inspect_si_path chain_unchanged: popf jmp DWORD PTR cs:[old_int21] inspect_dx_path: push ax push bx push cx push si push di mov si, dx mov di, offset scratch_one call copy_mapped_path pop di pop si pop cx pop bx pop ax jc dx_path_is_mapped jmp chain_unchanged dx_path_is_mapped: popf push bp mov bp, sp push ds push dx push cs pop ds mov dx, offset scratch_one mov byte ptr cs:[busy], 1 pushf call DWORD PTR cs:[old_int21] pushf pop word ptr ss:[bp + 6] mov byte ptr cs:[busy], 0 pop dx pop ds pop bp iret inspect_si_path: push ax push bx push cx push si push di mov di, offset scratch_one call copy_mapped_path pop di pop si pop cx pop bx pop ax jc si_path_is_mapped jmp chain_unchanged si_path_is_mapped: popf push bp mov bp, sp push ds push si push cs pop ds mov si, offset scratch_one mov byte ptr cs:[busy], 1 pushf call DWORD PTR cs:[old_int21] pushf pop word ptr ss:[bp + 6] mov byte ptr cs:[busy], 0 pop si pop ds pop bp iret inspect_rename_paths: mov byte ptr cs:[source_mapped], 0 mov byte ptr cs:[target_mapped], 0 push ax push bx push cx push si push di push bp mov bp, di /* preserve caller's ES:DI target offset */ mov si, dx mov di, offset scratch_one call copy_mapped_path jnc inspect_rename_target mov byte ptr cs:[source_mapped], 1 inspect_rename_target: push ds push es pop ds mov si, bp mov di, offset scratch_two call copy_mapped_path pop ds jnc rename_copy_done mov byte ptr cs:[target_mapped], 1 rename_copy_done: pop bp pop di pop si pop cx pop bx pop ax cmp byte ptr cs:[source_mapped], 0 jne rename_has_mapping cmp byte ptr cs:[target_mapped], 0 je chain_unchanged rename_has_mapping: popf push bp mov bp, sp push ds push dx push es push di cmp byte ptr cs:[source_mapped], 0 je rename_source_ready push cs pop ds mov dx, offset scratch_one rename_source_ready: cmp byte ptr cs:[target_mapped], 0 je rename_target_ready push cs pop es mov di, offset scratch_two rename_target_ready: mov byte ptr cs:[busy], 1 pushf call DWORD PTR cs:[old_int21] pushf pop word ptr ss:[bp + 6] mov byte ptr cs:[busy], 0 pop di pop es pop dx pop ds pop bp iret /* DS:SI = caller path, DI = resident scratch destination. CF=1 if copied. */ copy_mapped_path: push ax push bx push cx push si push di cmp byte ptr [si + 1], ':' jne copy_failed mov al, byte ptr [si] and al, 0xdf cmp al, 'A' jb copy_failed cmp al, 'Z' ja copy_failed sub al, 'A' xor bx, bx mov bl, al mov ah, byte ptr cs:[drive_map + bx] test ah, ah jz copy_failed mov byte ptr cs:[di], ah inc di inc si mov cx, SCRATCH_BYTES - 1 copy_loop: lodsb mov byte ptr cs:[di], al inc di test al, al jz copy_succeeded loop copy_loop copy_failed: pop di pop si pop cx pop bx pop ax clc ret copy_succeeded: pop di pop si pop cx pop bx pop ax stc ret old_int21: .word 0, 0 busy: .byte 0 source_mapped: .byte 0 target_mapped: .byte 0 drive_map: .fill 26, 1, 0 scratch_one: .fill SCRATCH_BYTES, 1, 0 scratch_two: .fill SCRATCH_BYTES, 1, 0 resident_end: