A frog crosses a road and a river to reach five homes at the top. It is the 1981 arcade game, cut down to eight kilobytes.
The screen is built from label lists (0x4A04 and four more), and it waits for one of four keys:
| key | players | controls |
|---|---|---|
| 1 | one | joystick |
| 2 | two | joystick |
| 3 | one | keyboard |
| 4 | two | keyboard |
0xE007 holds which one: 1 reads PSG register 14 (the joystick), 0 reads row 8 of the keyboard (the cursor keys, translated through the table at 0x49B7).
Before the title, and only the first time, there is a short presentation: a frog climbing the screen a row at a time from row 20 up to row 3 (0x427F). Pressing any of the four keys cuts it short.
The playfield is 22 columns wide, from column 2 to column 23; the rest of the screen is the scoreboard down the right-hand side, from column 25.
| rows | |
|---|---|
| 0 | the five homes, four characters each, at columns 4, 8, 12, 16 and 20 |
| 1–2 | the border below them |
| 3, 5, 7, 9 | the river: logs, turtles and the crocodile |
| 11–12 | the median, one character repeated 22 times |
| 13, 15, 17, 19 | the road: four kinds of vehicle |
| 21–23 | the bank the frog starts from |
Up and down move 16 pixels — a whole row. Left and right move only 8, half a cell. Each jump changes the drawing: 0x04 facing up, 0x10 down, 0x1C left and 0x24 right.
The frog is two sprites. While a jump lasts, the second one stays behind in the cell it left.
Ten points for every jump forward (ld bc,0x0010, in BCD). Jumping backwards gives nothing.
Each of the five has its own bit in 0xE080. Landing on one that is already taken is not a bounce and not a warning: 0x528F reads the bit and, if it was set, jumps straight to the death routine.
Filling all five ends the stage. Whatever is left on the clock is cashed in at ten points a unit, one every two frames.
The time starts at 0x96 and drops by one every 20 frames. The whole count is BCD —0x5104 subtracts with add a,099h / daa, and the thresholds are compared as cp 060h and cp 032h— so that 0x96 is ninety-six units, not 150: at 20 frames each, 32 seconds on a 60 Hz machine. Running out kills the frog.
Two things happen along the way that are easy to miss: at 0x60 four objects in row 15 speed up, once per stage; and at 0x32 the character the time bar is drawn with changes colour.
Three lives. The extra life threshold lives in 0xE005 and is compared against the high byte of the score, which counts tens of thousands: it starts at 1 and goes up by 5, so the first extra life comes at 10,000 and the rest every 50,000.
Only seven lives are ever drawn, however many there are.