Zilog Z80195 - Manual (Page 25)

Z80185/195 D EVELOPMENT K IT
U SER ' S M ANUAL
3-5 UM951800100
Z ILOG
Alter Memory [A]
Use the Alter Memory command to modify bytes in memory. The display shows the address and
current data. When a byte is entered, it is written to the address shown, and the next higher or lower
address is displayed. The monitor program does not write to the location when entering a terminating
value without the hex value before it. Terminating is as follows:
Does not store a preceding value and returns to the command prompt.
. Stores a preceding value if any and returns to the command prompt.
= Stores a preceding value if any, stays at the same location, re-reads and re-
displays.
CR,tab,Space,+,> Stores a preceding value if any, and go to the next location.
-,<,^ Stores a preceding value if any, and go to previous location.
Example:
Z80185 >Alter Memory starting at: 8000
8000 39 : bb
8001 20 : aa
8002 FF : ^
8001 AA :
Z80185 >
Set or Show Breakpoints [B]
A break point is a special status you can associate with a memory address in RAM, so that when you
set a program running with a Go-To-Program G command, and the execution comes to that address
at the start of the instruction, it will stop running and return to the monitor command prompt. Up to eight
(8) breakpoints can be set at one time. The code at each break address is replaced with the RST 28H
(hex EF). This opcode will bring the control back to the monitor if execution reaches the breakpoint.
The user can code RST 28H instruction right into the program, at the end of the program, and/or at
any point the user needs the control to come back to the monitor. In the case of this “hard-coded”
RST 28H, the monitor program simply increments the address to the following instruction before it
starts execution. If a breakpoint is set at the starting address in a G command, the monitor program
will set one single-steps over the first instruction, then sets all the breakpoints and goes. The monitor
also checks to see whether a requested breakpoint is in fact in RAM (where it must be).
Note: Breakpoints must be set at the first byte of an instruction. (Instruction starting points can be
determined by doing a command U.) Breakpoints that are randomly set (or breakpoints that are set
in the middle of an instruction) may cause faulty operation of the instruction.
(Example of the "Set or Show Breakpoints" command is shown on the next page.)