The cartridge

32,768 bytes mapped into pages 1 and 2, that is, 0x4000 to 0xBFFF.

The header

The first ten bytes are the MSX's AB header:

41 42 10 40 00 00 00 00 00 00
'A''B' INIT  STATEMENT  DEVICE  TEXT

It only declares INIT, at 0x4010. STATEMENT, DEVICE and TEXT are zero, and that is precisely what separates this cartridge from the 1984 Hole in One: that one registered a BASIC statement — CALL GOLF — and could therefore load courses from tape. This one does not. Outside courses get in another way: through the editor.

The layout

from	to	what
0x4000	0x4010	the AB header
0x4010	0x5009	boot, menu, scorecard and the round loop
0x5009	0x5175	the thirty-five golfers and their tables
0x5175	0x52DE	the title screen's texts
0x52DE	0x63E9	the shot: aim, bars, flight and physics
0x63E9	0x6603	terrain, arithmetic and the trigonometry tables
0x6603	0x6C0D	screen, interrupt, joystick and the PSG player
0x6C0D	0x6F49	the tone table and the twelve tracks
0x6F49	0x716B	the hole builder
0x716B	0x9DDD	the two courses: 36 pointers, 36 scripts and the green
0x9DDD	0x9F85	the close-up view and the swing
0x9F85	0xAEA3	everything that decompresses into VRAM
0xAEA3	0xC000	the course editor and the tape code

In numbers: 14,186 bytes of traced code and 18,582 of named data. None unaccounted for.

How it boots

init (0x4010) does the usual and one thing more. First it works out which slot it is in itself — RSLREG, the subslot table at 0xFCC1 — and stores the result at 0xFEDB. It will need that note much later, when the tape routine swaps page 1 for the BASIC ROM and has to put it back.

Then it pages its own second half into page 2 with ENASLT, clears 0xC002–0xF300 in one go, leaves QUEEN SIDE as the default course and the TOP at 0x12 — eighteen, one over par on every hole — starts the PSG, hooks the interrupt and goes off to the attract mode.

The interrupt, across slots

0x6603 does not put a jp into H.TIMI: it puts an inter-slot call.

ld a,0f7h        ; 0xF7 is RST 30h, that is, CALSLT
ld (0fd9fh),a
ld a,(0fedbh)    ; the slot, the one init noted down
ld (0fda0h),a
ld hl,0665bh     ; and the address
ld (0fda1h),hl

It has to be that way because the handler lives inside the cartridge, and when the interrupt fires there is no guarantee the cartridge is paged in. The handler (0x665B) does two things: it marks that a frame has passed, and it gives the PSG player one step.

Three screens in VRAM

The MSX's SCREEN 2 has one name table, but VDP register 2 says where it is. This cartridge builds three and flips that register:

address	what it is	how you get there
0x1800	the playing screen	the default, R2=0x06
0x1C00	the eighteen-hole scorecard	0x678B, xor 1
0x3C00	the tournament leaderboard	0x67CC, xor 9

Pressing F1 or F2 mid-round redraws nothing. It shows another screen that was already built, and letting go returns to the previous one.

The three thirds, from a single table

SCREEN 2 is three independent thirds, and for all three to hold the same thing you have to write it three times. 0x66D4 makes the six calls — three of patterns and three of colour — and all three of each kind decompress the same source: 2 KB of cartridge for 6 KB of table.

And there is a second loading mode hidden in the same place. 0x66D4 begins with an or 0AFh, which is the two bytes F6 AF. Entering at 0x66D5 — the second byte of that instruction — what runs is a bare xor a: A goes to zero, Z is set and the six calls load only the 360-byte tail of each third, which is tiles 0xD3 to 0xFF.

And those forty-five tiles are the title wordmark. The same ones that in play are the tee, the green and the flag. The cartridge swaps them by entering one instruction or the next, and (0xCA40) records which of the two sets is in place: that is why pressing F1 at the menu loads the playing tiles so the scorecard can be drawn, and then puts the wordmark's back.

The eleven compressed blocks

There are two decompressors, told apart by their escape byte:

• 0x6AFA, run-length: 0xA0–0xAF means (n & 0x0F) + 2 copies of the next byte.
• 0x6ABD, pairs: 0x00–0x0F means (n & 0x0F) + 1 rounds of a pair of alternating bytes. This is the one for SCREEN 2's colour tables, where bytes come in pairs.

Both stop when the destination reaches its limit, not when the input runs out. So where each block ends is written down nowhere: you find out by decompressing it. Do that, and the eleven fit one after another without a byte to spare, from 0x9F85 to 0xAEA3. It is the best evidence the format is understood, and there is a test watching it.