6502 : Program Library

ROMLIB4

Our subroutine library ROMLIB3 is saved in memory at $E000.  There is nothing special about the code, we could store programs, program snippets or data in the library.  In fact I decided to store programs in ROMLIB4 starting at $E400.

As a first example I stored echo.s, my previous simple command line in ROMLIB4.  Following Dirk Grappendorfs monitor I added a command "jhhhh" to allow the user to jump in to a program.  For the first example "je400" causes the echo program to be invoked with its simple command line.  I added an "e" option to echo which exits back to cmdline with a "jmp $8000" causing it to reinitialise.

Previously I have written two diagnostic programs to show messages on terminal and LCD without using RAM.  I added these to the program library so they can be started from the monitor.  Alternatively they can be started by changing the contents of the reset vector at FFFC, FFFD or writing a short program, loaded at $8000 with a statment "jmp $E4xx".

As with the subroutine library I need to be very careful that labels are unique.  For programs there is more likely to be a clash as one program is often used as the basis for the next.  To make life simple I add a suffix, 1,2,3,4,... to the labels in successive programs to make them unique.

Instead of jumping to a program address from the monitor we would like to do a jump subroutine to an address in the subroutine libary with a call "shhhh", for example "sE1E2" to display registers and return to the monitor.  The "j" jump option used the "jmp (indirect)" opcode but the 6502 / 65C02 doesnt have a "jsr (indirect)" opcode.  I simulate this by putting a return address on the stack and jumping to the subroutine as shown below.

TESTBED

I can now add my cmdline.s to the program library (its start address is $E5D7).  I start a new program testbed.s which starts with the instruction "jmp $E5D7" which is a 3 byte opcode at $8000.  The program itself continues at $8003 and in fact I display "#" as a terminal prompt followed by options which I want to test.  One of the options is "e" which takes us back to the monitor.

Now when I compile and load the program I see my cmdline monitor.  I can then issue the command "j8003" which takes me to the "#" prompt and allows me to run functions I am testing.  Typing "e" takes me back to the monitor.

The testbed program is only about 100 bytes so it assembles really quickly.  The program logic is in ROMLIB4/cmdline.s and subroutines are in romlib3.  This gives me an excellent testing environment.