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These calls are fairly friendly as far as register corruption is concerned. OS_Out writes the character held in register A and forces Fc = 0 before returning with no other register or flag changes. GN_Nln changes no registers and returns Fc = 0 unless there is an error. GN_Sop and GN_Soe expect HL (or BHL) to point to a null-terminated string, and on exit HL (or BHL) will point to the null, with other registers remaining the same, unless there is an error. Errors can occur if the standard output device has been redirected to somewhere other than the screen. In the case of errors Fc = 1 and register A will contain an error code. If you are not doing any redirection yourself then these kind of error situations can be largely ignored, since if the user is doing bizarre stream rebindings it is not your responsibility what happens. Examples of these routines in use can be seen in the "Input and the keyboard decoder" section.
In using the Z88 you may well have come across a "Page Wait" message which appears on the left hand side of the screen in place of the application name and list of topics. This message can be generated by an application (it is not done automatically) when scrolling output is being sent to the screen. The actual system call involved is OS_Sr with register A = SR_PWT. This call produces the "Page Wait" message, waits for a key, restores the application name and topic list and then returns to the application. Because at the core of this routine there is a keyread, application writers must be careful to check for pre-emption codes, especially RC_QUIT. Although the writer may think about "Page Wait" as primarily an output function, it has all the characteristics of an input function. However, it only returns <BACKSPACE> (ASCII 8) and all relevant error codes like RC_QUIT and RC_DRAW.
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in fact, any graphic effects used by the built-in software with the single exception of the map. This is accessed more directly through the system calls, in particular OS_Map described in "Miscellaneous useful routines". The Standard library have also supplied utilities to plot, draw lines and sprites in the map area, which easily can be linked with application code using the Z80 Module Assembler Mpm.
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Application windows may be preserved by using the system call OS_Sr, A=SR_SUS. This is a lazy method and consumes 2K of continuous memory - don't use use it unless strictly necessary. Further, only the screen area starting from (10,0) to the right edge (before the OZ window) are preserved. Windows beyond that has to be redrawn explicitly. In any case, application windows must be redrawn when your application is re-entered (otherwise your are faced with a blank screen). If your application uses a window that overlaps the system's topic window you must also re-draw your window after an executed command, pressing <MENU> or the <HELP> key.
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There are three calls which can be used to obtain information about the current state of the screen and they are all accessed via OS_Nq which takes a reason code in the BC register. A fuller description is given in "Miscellaneous useful routines", but the reason codes of interest are:
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