Apple IIgs #16
Notes on Background Printing

Revised by Mike Askins (November 1988)
Written by Mike Askins (June 1987)

This Technical Note attempts to pinpoint some of the common problems people encounter when using background printing as available through the serial firmware.

Calling Sequence

Init call
Starts the serial firmware
Specifies a buffer to place data to be printed
Places data in buffer ( amount < buffer size)
Starts the background printing process

Correctly Making the SendQueue Call

The Apple IIGS Firmware Reference incorrectly documents the parameters you pass to SendQueue. The correct specification of the recharge address does not correspond to the standard method of passing a full 32-bit address. Set the parameters as follows:

    Launches background printing.

        CmdList    DFB $04                       ;Parameter Count
                   DFB$18                        ;Command Code
                   DW $00                        ;Result Code (output)
                   DW  DataLength
                   DFB RechargeAddress (bank)
                   DFB RechargeAddress (high)
                   DFB RechargeAddress (low)
                   DFB $00

Using the Default Buffer

You can use the area which the firmware reserves for transparent buffering to place data for background printing. This is advantageous since the firmware calls the Memory Manager to allocate space for the buffer (you must allocate the space from the Memory Manager if you use the SetOutBuff call to set up a buffer).

To use the serial firmware's buffer, you must first enable buffering by initializing the port with PINIT and sending it the string "^IBE" with PWRITE. Once you enable buffering, call GetOutBuff to find the size and location of the buffer, then place your data (buffersize - 1) in the buffer and call SendQueue.

Data Size

Make sure that the amount of data you place in the buffer is at least one byte less than the size of the buffer since the firmware uses one byte of the buffer for bookkeeping purposes; if you place too much data in the buffer, it will continually print the buffer's contents and never call your recharge routine.

The Recharge Routine

You should treat the recharge routine as an interrupt handler and execute it at interrupt time. Interrupts are disabled at this time, and it is illegal to enable them within the recharge routine. Like all interrupt handlers, the recharge routine should take care of its business as quickly as possible then exit; any excessive delays cause interrupt dependent processes (e.g., AppleTalk) to fail. You should also remember that most of the system code is non-reentrant; you should use the Scheduler when calling system code which may have been running when the serial interrupt that invoked the recharge routine occurred.

The serial firmware is not generally reentrant and does not interact with the Scheduler. If you want to make serial firmware calls (through $C1xx, $C2xx) from your recharge routine, you must preserve MSLOT (the byte at $0007F8) across those calls. Be aware that any non-recharge code must not make calls to the serial firmware that will disrupt the background printing process; sending the string "^BD" (disable buffering command), for example, is guaranteed to confuse a running background printing process.

Further Reference

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