6502 : Library and Shell User Interface

Library

We now have a number of subroutines for simple tasks which will be useful in a variety of programs.  It doesn't make sense to process them every time a program is assembled.  It is much better to save them permanently in an area of ROM, that is to say a library.

We start with a modified assembly procedure which saves object code starting at address $E000.  The library source is a set of subroutines (in alphabetic order) which I call ROMLIB3 (because it was my third attempt!).  Once these have been assembled and saved in ROM we look at the assembly listing to find the start address of each subroutine and add the address as a symbol in our program.  By convention I make the symbols upper case.  I can then delete the source subroutine from the program and replace the jump subroutine name with a symbol.

In this way we can use any or all subroutines in the library simply by adding a symbol table containing start addresses to a program. 

The upload sketch to store hex in ROM can only process about 30-50 bytes per second.  Our programs were increasing in size towards 1KB and we are now down to about 400 bytes so load time is halved to about 10s.

Initial library contents include:
Terminal i/o: getc, init, putc, putstr, hex_digit, hex_digits, registers (to show current contents)
LCD output: init_VIA, init_LCD,  clear, instruction (control command), putc, wait (until ready), hex_digit, hex_digits, putstr

We do have to be careful with our organisation when using libraries.  It is not permissable to change the contents of a library member as this will mean that the start addresses of all subsquent subroutines change and in turn the symbol table in each program needs to be amended and the program re-assembed.  Instead, if a subroutine must be altered, its labels are all prefixed with "OLD" and the new version of the subroutine added to the bottom of the list.  The latest program can then use the new version whilst previous projects remain unchanged.  

Shell

Rather than load a program which executes a single function it is useful to have a "monitor" program.  In fact this is a simple "shell" program such as bash, which is loaded when the user signs on and allows them to carry out various function.

Our shell starts of simple as shown in the picture below.  It shows a title on the terminal, in this case "Ava go" and a ">" prompt.  User input is a single letter, valid choices are l,n,o,p and Return.  The "l" option simply prints an upper case "L".  I find this very useful to check if input is working. "n" displays a hex string on the LCD screen, "o" is a memory dump progrm (wip), "p" tests the newline and "Return" shows a prompt.

We have now moved on from building hardware and writing programs which utilise it to a computer with a user interface and subroutines available to make the programmers life easier.

In the next episode we will expand our shell program to help us check on the current status of the computer.