z80 » Advanced
Morphic code is code that edits other code. This is such a broad definition
because it changes, as the name hints. You can have a routine that puts
a return instruction between some code to return early. It would
load the value for
$00) so it wouldn't move any
code around affecting all addresses after. Morphic code isn't
used that often. One time it is used is for random number generators. Here's
an example of a random number generator that edits a constant every time
it is run.
random_number_generator: push hl ;save hl ld a,r ;refresh register ; is increased after ; every instruction ; is executed ld hl,random_byte_addr ;address of byte ; we keep changing add a,(hl) ;add r to what's at hl adc a,a ;multiply by 2 and ;increase if carry ; was set last time ld (hl),a ;load the result back ; at hl pop hl ;get back hl saved ; from when we ; started the routine ret random_byte_addr: .db $00 ;byte we change
Now that code wasn't trully morphic. It more or less
resembled using variables. Morphic code is powerful when
used to modify other code as this next example shows. Upon
entering this routine, the byte at address
will contain the number of times we should loop using
djnz. We load that value into the byte loaded into
b using the Instruction
$) and start the loop.
loop_iteration_change: ld hl,loop_iteration ld a,(hl) ld hl,byte_to_change ld (hl),a byte_to_change = $ + 1 ;the $ would be ; the opcode byte ; telling the z80 ; to load the following ; byte into b ;$ + 1 would be the ; byte to be loaded ; into b ld b,$00 ;the $00 is changed ; from the 'ld (hl),a' ; code ld a,1 ;load a with 1 so we can ; start multiplying ; (really adding) loop: add a,a ;a*2=a djnz loop ret ;we now have a=2^b loop_iteration: .db $00 ;byte determined before ; routine starts
Let's reference a Temporary Trash variable within the program code.
ld a,69 ld (variable_addr),a ld a,$00 variable_addr =$-1
More from z80 » Advanced
All The Ports // APD // Assembler Directives // Entry Stack // User Fonts // IM 1 // IM 2 // Index, Shadow, and Other Registers // User Interrupt // Morphic Code // On-Off // Reading Keypresses from Port // Shift and Rotate // Simulating Key Presses // Sound // Square-Root Programs // System Flags of TI-OS