push ds
        mov ax, 40h
        mov ds, ax
        mov ax, word [82h]      ; end of circular keypress buffer
        mov dx, word [80h]      ; start of circular buffer
        test ax, ax
        jz .forcekeybuffer
        test dx, dx
        jz .forcekeybuffer
        mov bx, ax
        sub bx, dx              ; cmp end, start
        jbe .forcekeybuffer     ; below or equal is invalid -->
        test bl, 1              ; even amount of bytes ?
        jnz .forcekeybuffer     ; no, invalid -->
        mov bx, word [1Ah]      ; current head of circular buffer
        cmp bx, ax
        jae .forcekeybuffer
        sub bx, dx
        jb .forcekeybuffer
        test bl, 1
        jnz .forcekeybuffer     ; invalid -->
        mov bx, word [1Ch]      ; current tail of circular buffer
        cmp bx, ax
        jae .forcekeybuffer
        sub bx, dx
        jb .forcekeybuffer
        test bl, 1
        jz @F                   ; valid -->
.forcekeybuffer:
        pop ds
        mov word [io_end_buffer], 3Eh
        mov word [io_start_buffer], 1Eh
        db __TEST_IMM8          ; (skip pop)
@@:
        pop ds


.check_rombios_buffer:
        push bx
        push dx
        mov ax, 40h             ; dual mode segment/selector
         push ax
        mov ax, word [io_end_buffer]
        mov dx, word [io_start_buffer]
         pop ds
        test ax, ax
        jnz @F
        mov ax, word [82h]      ; end of circular keypress buffer
@@:
        test dx, dx
        jnz @F
        mov dx, word [80h]      ; start of circular buffer
@@:
        mov bx, ax
        sub bx, dx              ; cmp end, start
        jbe .ret_dx_bx          ; invalid -->
        test bl, 1              ; even amount of bytes ?
        jnz .ret_dx_bx          ; invalid -->
        mov bx, word [1Ah]      ; current head of circular buffer
.loop:
        cmp bx, word [1Ch]      ; equal to current tail ?
        je .ret_dx_bx           ; yes, all entries checked -->
        cmp byte [bx], 3        ; is it Ctrl-C ?
        je handle_ctrl_c        ; yes, handle -->
        inc bx
        inc bx                  ; -> next entry
        cmp bx, ax              ; at end of buffer ?
        jb .loop                ; no, loop -->
        ja .ret_dx_bx           ; invalid -->
        mov bx, dx              ; reset to start of buffer
        jmp .loop               ; then loop -->

.ret_dx_bx:
        pop dx
        pop bx