A Z- 80 Cycle Waster

The Z- 80 code is in the comments alongside equivalent C code. The C program is self-contained and runs an exhaustive test verifying that waithL() always uses h 256 + L + 100 cycles. observe that the Jr conditional branch instructions take extra time if the branch is taken. Those time differences along with looping are used to expand the subroutine’s running time in proportion to the requested number of cycles.

// Z- 80 registers are akin to C global variables.

unsigned char A, H, L; unsigned char C, Z;

int cycles;

void t(int n)
{
cycles += n;

} void waitA();

/* Cycle Cost C code
Z- 80 code */
void wait256()
{
wHL256:
}
H--; Z = H != 0;
A = 127;
waitA();

t( 4); t( 7); t( 17);

// DEC H
// LD A,127
// CALL wA
void waitHL()
{
wHL:
t( 4);
t( 4);
t( 7);
t( 5);

H++; Z = H == 0;

H--; Z = H == 0;
if (!Z) {
// INC H
// DEC H
// JR NZ,wHL256

t( 4); t(0);

} A = L; waitA();

// LD A,L
}
void waitA()
{
wA:
t( 4);

C = A & 1; // RRCA A = (A << 7) | (A >> 1); if (C) { // JR C,have1

goto have1;

t( 7); t( 5);

} t( 4);

// NOP
have1:
t( 4);

t( 7); t( 5);

C = A & 1; // RRCA A = (A << 7) | (A >> 1); if (!C) { // JR NC,no2

goto no2;

}
if (!C) {
t( 7);
t( 5);

no2: t( 4);

t( 7); t( 5);

C = A & 1; // RRCA A = (A << 7) | (A >> 1); if (!C) { // JR NC,no4

goto no4;

}
if (!C) {
t( 5);
t( 6);

// NOP
no4:
t( 4);

t( 7); t( 5);

C = A & 1; // RRCA A = (A << 7) | (A >> 1); if (!C) { // JR NC,no8

goto no8;

t( 13);

}
/* *0 = A; */
// LD (0),A
no8:
t( 7);

A &= 15;

Z = A == 0;
if (Z) {
// AND A, 15

t( 5); t( 6);

}
// RET Z
wait16:
t( 4);
t( 7);
t( 5);

A--; Z = A == 0;
if (!Z) {
// DEC A
// JR NZ,wait16

}
if (Z) {
t( 5);
t( 6);

#include <stdio.h> int main(int argc, char *argv[])

if (cycles != hl + 100)

{ printf(“Blew it on %d (got %d instead of %d)\n”, hl, cycles, hl + 100);

} } return 0;