Comparing - The CP Instruction
The CP instruction stands for compare. It takes the general form
cp ValueToCompareWith and compares the value of the a register with
ValueToCompareWith, which can be any 8 bit register, an 8 bit number or (hl). So:
cp 7 cp b cp e cp (hl) cp %11010111
and so on. What the CP instruction actually does is substracts ValueToCompareWith from the value of the a register, but it doesn't store the result anywhere or affect the value of either the a register or ValueToCompareWith. What it does do is affect the flags.
Flags - The f Register
The f register is sometimes called the flag register. It can't be used in maths, logic or any other 'useful' instruction (it can be pushed and popped from the stack when paired with a to give the af register pair). However, many instructions affect the value of f. Most of the bits in f have a meaning, for example one of the bits is set if there has been a carry and reset if there hasn't.
Conditional Calls and Jumps
The f register lets us use 'if..else' statements in assembly. There is another form
of the CALL, JR and JP instructions, which is call condition,address (or
jp condition,address or jr condition,address). The address
part remains the same, and condition is one of the following (there are other
possibilities, but they don't all work for JR):
| Condition | Meaning |
|---|---|
| Z | Zero - the instruction is only executed if the result was zero (in other words if the value were equal). |
| NZ | Not Zero - the instruction is only executed if the result was not zero (in other words if the value were not equal). |
| C | Carry - the instruction is only executed if there was a carry (in other words if a was less than the argument). |
| NC | No (or Not) Carry - the instruction is only executed if there was a not carry (in other words if a was greater than or equal to the argument). |
Note that you can also do ret condition.
A Useful Program (Sort Of)
So you can now finally write a useful program (or at least you can once I've told you
what call _getkey does). The _getkey ROM routine pauses the calculator
until a key is pressed, and then returns the value of that key in the a register. It
also trashes a few other registers, so if you need them don't forget to push them
onto the stack. The number returned in the a register isn't the same as the one used
in the built in language, it's a bit more advanced. There are equates defined for
the different keys, so rather than doing cp 7 you can do cp
kExit (for some reason there's no underscore). If you need a list of the
equates for the keys they're in a file called "asm86.h" which you'll find in
whatever directory you installed Assembly Studio in in a subdirectory called
"include". They all start with a small k - the ones with a capital k are something
else.
Program Specification
The program should clear the screen. It should then wait for a key to be pressed. If the key is [EXIT] then the program should quit. If the key is [ENTER] then the program should display a star (character 42) on the screen and go onto the next line. If it is any other key the program should do nothing. The program should then wait for another key and so on.
The Code
Try and work out your own code before copying the one below. Remember, your code will probably be different from mine because I've used a few shortcuts, but as long as it works that's fine.
Source: getkey1.asm
Compiled: getkey1.86p
#include "ti86asm.inc" .org _asm_exec_ram call _clrScrn ; Clears the screen (duuh!) call _homeup ; Cursor to top left LoopStart: call _getkey ; Wait for a key to be pressed cp kExit ; Compare it to kExit - [EXIT] key ret z ; If they're equal return to the OS cp kEnter ; Otherwise compare it to kEnter - [ENTER] key jr nz,LoopStart ; If it's not enter go and get another key ; If it is enter... ld a,42 ; a = 42 (ASCII character '*') call _putc ; Display charater in a call _newline ; Go onto a new line jr LoopStart ; Then go back to the start of the loop .end ; Tell the assembler to stop
Screenshots
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| One star | Four stars |
