Technical information

NewBIOS Reference Serial Solutions

Meaning._________

Service B3H is used to read the status of the buffers

associated with a port, and to control those buffers. NewBIOS

itself does not use these values but the NewCOM.SYS device

driver does.

The buffer status of a port which has been opened by

NewCOM but for polled rather than buffered i/o is reported by

subservices 00H and 01H as a port with 1-byte input and output

buffers, the chip registers. This is to allow software to be able to

make the same tests of the ’buffers’, whether or not they are

real. There are however some differences. Because the output

’output buffer’ will still be sent if the output handshake goes

false. Then, even though there appears to be a byte free in the

output buffer, writes to it will be blocked. These ’buffers’ cannot

be flushed or halted.

Port which have not been opened are reported as having

halted buffers which are zero bytes long.

The disable buffer functions simply prevent data from

moving between the buffer and the serial chip by disabling the

interrupt responsible for that. Reads and writes to the port can

still be performed. Bytes coming in to the port will be lost if the

input buffer is disabled. By default both buffers are enabled.

Manipulating buffer contents using subservices 08h & 09h.

mov ah,0B3h

mov al,08h ;get input buffer addresses

int 14h

mov es,ax ;buffer is in this segment

mov di,ds:[bx]

;save the contents of the tail ptr here for later

;

;calculate amount of data in the buffer

mov ax,ds:[cx] ;get the contents of the head ptr

mov bx,ds:[bx] ;get the contents of the tail ptr

sub ax,bx ;take tail away from head

Chapter 5 Page 105