Debug General Description
	The Debug program will allow the programmer to analyze the
 operation and effects of a Machine Language Program (MCL). This is
 particularly valuable in getting a program to work, that is in
 debugging the program that you have written. The Debug program can
 also be used to analyze the operation of resident ROM programs. It may also be an aid in understanding the operation
of the M-100 microcomputer.

Operation:
  DEBUG.100 is a Basic language program that employs a Machine Language Program (DEBUG.CO) for speed. This MCL 
occupys memory from
 60672 to 63959 (MAXRAM) . The DEBUG.CO is created by loading and running DEBUGM.100.
	The MCL under test is assembled by the Assembler from this SIG 
or by another. It must also be loaded into Model-100 memory by poking
 or by other means. The best place is probably just below the
 DEBUG.CO, just below location 60672 ($ED00). Once your test MCL is loaded the DEBUG.100 program is RUN. 

Model-100 Data Display 
The top line is a mode status line and it shows the state of the following modes. These are described below.
	Subroutine bypass mode. "SUBF"
	Multiple step mode.     "MSTP"
	Breakpoint Active mode  "BKPT"

The Model-100 screen will show the contents of the following registers: 
	PC (program counter)
	A Register
	B and C Registers
	D and E Registers
	H and L Registers
	S Register relative to its initial value
     Each system status flag is shown by letter if its value is true.
The flags are 	"M" = minus
		"Z" = zero
		"H" = half carry
		"P" = parity
		"C" = carry
For illustration, if carry and zero are set then  [ Z  C]  will be shown. 
	Below the register contents is the disassembled instruction that occurs at the memory location designated by 
the Program Counter (PC value).
	Further below is a listed reference memory pointer and a display of five consecutive memory locations 
starting with that pointer value.
The ASCII character interpretation of those five locations is also shown.
	The bottom line is used for prompts and for command/data entry. The initial prompt is "RXS^.<>+*QE" which is 
explained below. 

Data Entry - working registers.
Data is entered into a register by pressing "R". The Model-100 will respond with this prompt "ACBEDLHFPM". At this 
time enter a register letter or "F" for "Flags". After a register letter then enter a 2 digit hexadecimal value for 
registers A to H and a 4 digit hexadecimal value for the PC or Memory pointer. After "F" enter "M", "Z", "H", "P", OR
"C" to toggle that flag.

Changing the PC
The Program Counter (PC) is changed by typing "P" after the register prompt and then entering a 4-digit hexadecimal 
value. The PC can be advanced to the next statement (without execution) by typing "+". The program counter also may 
be changed by execution as described below. 

Changing the Memory Pointer.
The Memory pointer is changed by typing "M" after the register prompt and then entering a 4-digit hexadecimal value. 
The Memory pointer may be advanced one location by typing ">" or moved back one location by tuping "<". The contents 
of the 5 displayed memory locations are useful to show the stored results of program execution. This is often a 
particular RAM location, although if desired the memory pointer may be locked-on to the PC by typing "^". It is 
toggled back to the original value by typing "^". 

Changing the contents at the Memory pointer
The value referenced by the memory pointer is changed by typing "." after the main prompt and then entering a 2-digit
hexidecimal value.

Changing the Debug Modes.
 The modes may be changed by typing "*" and then the "SUBF" mode or the "MSTP" mode or the "BKPT" may be toggled. If 
The Breakpoint is activated then you must enter a 4 digit hexadecimal value. This is the location where the computer 
will breakout of its execution and display its registers..

Executing the Program.
The MCL may be executed from the PC location in several ways at the main prompt. Type "X" and the program will 
execute from the PC location. If teh "BKTP" is set it will continue until the breakpoint or indefinetely if no 
breakpoint is encountered. After the breakpoint, the current status of all registers, flags and the five memory 
locations are shown. If "X" is typed when "BKPT" is inactive then the program will execute a subroutine starting at 
the PC. (until a return "RET" at the proper nesting level). If "S" is typed then only one instruction is executed.
Subroutines are often tedious to step through especially if they have already been checked out. If "MSTP" is set then
after typing "S" you must also tyoe the number of steps desired. DEBUG will step around them (executing without pause
) is the "SUBR" mode is set. Note that the all execution functions including step and breakpoint and execute (X) 
operate on ROM MCL programs as well as RAM.  

General Problems: Operating a new and untried Machine Language Program may easily erase executive RAM in the Model-
100. This may necessitate a total restart by momentarily removing power from the RAM (the switch on the bottom) or 
typing Cntl-Shift-Break while turning power on. This is not serious if it doesn't happen often and if your source 
programs can easily be loaded.O