How it works

The six things, one by one, with an address behind every claim.

A unit's map

The engine keeps 256 unit slots, in parallel 256-byte arrays indexed by unit number:

address	what it holds
`0xB900+n`	column (X) on the map
`0xBA00+n`	row (Y)
`0xBD00+n`	type, side and flags; bit 4 = carries the Ring
`0xC000+n`	low nibble Valioso, high nibble Hábil
`0xC100+n`	low nibble Duro, high nibble Bravo
`0xC200+n`	Enérgico (spent by walking)
`0xC300+n`	Decidido (goes up one every month)

Yours are slots 0x00-0x77 except 0x16 and 0x17; the other side's are those two and everything from 0x78 up. The Ring-bearer is the first slot with bit 4 of 0xBD00 set: in a new game, Frodo, unit 0x05.

And the map lives from 0xCC00, column by column: its 0x33CC bytes are 130 columns of 0x66, so cell (x, y) sits at 0xCC00 + (x+1)*102 + (y+1).

1 · Enemy units become visible

In each map cell, bit 7 means "someone is here", and it is what makes the game draw the 2×2 silhouette. RECENTRA_EL_MAPA (0x7FAC) sets it: it clears bit 7 across the whole map and re-plants it unit by unit. But the loop stops at 0x78:

7FB0  ld c,000h        ; starts at unit 0
7FB2  ...              ; plants bit 7 in unit C's cell
7FCC  set 7,(hl)       ; "someone is here"
7FCE  inc c
7FCF  ld a,c
7FD0  cp 078h          ; <- stops exactly where the enemy begins
7FD2  jr nz,0x7FB2

One byte at 0x7FD1, from 0x78 to 0x00, and the loop walks all 256 slots. 0x7FC7 already skips the ones sitting at (0,0), so empty slots do no harm. Measured: 136 enemy units planted where zero were before.

Note. The opcode switch at 0x8982/0x8993 over 0x8AF3 — writing 0xD0 = ret nc or 0xD8 = ret c — does not decide this: it is the battle's side filter. That was checked before ruling it out.

2 · Every attribute shows its number

The sheet is assembled in a buffer at 0x7C17, 24 columns by 10 rows, and painted afterwards. Column 20 of each line has room for three digits.

The new routine lives at 0x6600 and a three-byte trampoline at 0x708A hooks it: where ARMA_LA_FICHA used to do ld hl,0x5FBD right before painting, it now does call 0x6600. The routine writes the six numbers and ends by redoing that same ld hl,0x5FBD, so the render is unchanged.

The values come from 0xC000-0xC300 for whichever unit the operand at 0x6EAD names, and the digits are written by ESCRIBE_A_EN_TRES_CIFRAS (0x7113), which is the game's own.

3 · The Ring's deadline

The clock (0x831B) counts ticks, days and months. Going from day 60 to 61:

8332  ld a,000h      ; the operand at 0x8333 is the COUNTDOWN of months
8334  dec a          ; 0x7F4F sets it to 255 when the game starts
8335  ld (08333h),a  ; one month less
8338  jp z,DERROTA   ; at zero, Sauron's screen
8340  ld hl,0853ah   ; "El Anillo corrompe al que lo usa."

That operand is what you have left before losing, and the game ties it to the Ring with its own message. There are other roads to DERROTA — losing the bearer, running the 0xC000 uses down in battle, handing the Ring to a unit with none — but that is the only one that is a deadline.

MARCA_AL_PORTADOR (0x6F6E) checks bit 4 of 0xBD00 and, if that unit carries the Ring, writes character 0x5F at 0x7C46, the last column of the sheet's second row. To its left, 0x7C43, three columns were free.

That ld a,05fh + ld (07c46h),a is now a call ANILLO_CON_PLAZO (0x666E), which places the ring the same way and also writes 0x8333 at 0x7C43.

And it saves BC, DE and HL. What follows in the sheet is call DESCRIBE_EL_DESTINO (0x6F7C), which relies on the HL it was handed: 0x7C27, where the ESCRIBE_A_EN_TRES_CIFRAS at 0x6F6B left it. Without saving it the sheet gets written somewhere else, with half the Fellowship's names on top of it. Tried, and you can see it.

4 · The Eye of Sauron

The game picks a cell's artwork by looking at its map byte and nothing else:

PINTA_LA_UNIDAD (0x7708):   or a          ; without bit 7 there is nothing
                            ret p
                            bit 6,a       ; cell with an order under way -> 0x11
                            ld a,011h
                            jr nz,ESTAMPA
                            ld a,015h     ; otherwise, the usual artwork

So when it draws it cannot know which side a unit belongs to. That has to go into the map byte itself, and for that we need a free bit. Bit 5 was free: zero uses across all 13,260 cells.

SIEMBRA_CON_BANDO (0x664C) replaces the call CELDA_DEL_MAPA + set 7,(hl) at 0x7FC9: it sets bit 7 as always and, if the unit is 0x78 or higher, bit 5 too.
DIBUJO_SEGUN_BANDO (0x6658) replaces the body of PINTA_LA_UNIDAD from 0x770A and looks at that bit 5 before anything else.

The icon is four new tiles at indices 111 to 114 of the 0x9E00 table, which were all zeros. Each tile is nine bytes: eight of artwork and a ZX Spectrum attribute glued behind.

The ZX's colour is per 8×8 cell, not per pixel, so the Eye gets two colours per quadrant. It uses them: it was repainted in dark red on the map's cream — attribute 0x3A, and 0x17 in the top-left quadrant, which runs the other way round — and the friendly icon became a blue shield. Both used to carry the same 0x38, black on white, and at a glance they looked too much alike.

5 · The text finishes its translation

Animagic's conversion translated the game half way: the map's place names stayed in English and three race names came across truncated. Nineteen strings change, and not one byte moves. What makes that possible is that the game keeps its text in four different shapes, and each one allows something different.

The place table, 0x7A5E. BUSCA_EL_SITIO (0x6E50) walks it. Each record is

[x][y][2 + width*rows][width<<4 | rows][text]

with no terminator: the length comes out of the third byte, which is also what you add to reach the next record. The fourth byte is the size of the signpost VENTANA_DEL_SITIO (0x6E2D) draws, and the text fills it whole, row by row — that is why Minas Tirith is twelve letters in 6×2 and Monte Gundabad sixteen in 8×2, with the spaces put in by hand. A new name has to measure exactly width × rows:

address	was	is	fits because
`0x7B34`	Bywater	Delagua	7×1, seven letters exactly
`0x7B28`	Buckland	LosGamos	8×1
`0x7B5D`	Far Downs	Quebradas	9×1
`0x7B4B`	Michel Delving	Cavada Grande	7×2: `Cavada` + `Grande`
`0x7BC7`	Grey Havens	Ptos Grises	6×2: `Ptos` + `Grises`
`0x7AA5`	Rivendell	Rivendel	9×1 to 8×1: one letter to spare
`0x7AB2`	Isenmouthe	Ga. Hierro	10×1
`0x7A79`	Morannon	Puerta N	8×1
`0x7B0D`	Dale	Valle	4×1 to 5×1, with the byte Rivendel lends it
`0x7B7F`	HelmsDeep	AbismHelm	5×2: `Abism` + `Helm`

The lists of packed strings, each ending with bit 7 set on its last letter. String N is reached by counting terminators from a base (SALTA_B_TEXTOS, 0x6E98). There are four: races in the plural (0x7D06), in the singular (0x7D39), the side labels (0x7D6A) and the adverbs (0x7D9A). Inside a list a string may change length as long as the total does not — and that is what pays for the longer words. Hum → Hombre is three bytes more and Elf → Elfo one more; there is no room behind, because 0x7D6A is a fixed address in the code. But Brujo → Mago is two bytes less, and Brujo appears twice in each list (races 0 and 7 are two kinds of wizard):

singular (0x7D3A, 44 bytes, races 0..8)
  Brujo 6  Nazgul 6  Hum 3     Elf 3   Enano 5  Orc 3  Hobbit 6  Brujo 6  Gollum 6  = 44
  Mago 4   Nazgul 6  Hombre 6  Elfo 4  Enano 5  Orc 3  Hobbit 6  Mago 4   Gollum 6  = 44

plural (0x7D07, 45 bytes, races 0..7)
  Brujos 7  Nazgul 6  Hum 3      Elfos 5  Enanos 7  Orcs 4  Hobbits 7  Brujo 6  = 45
  Magos 5   Nazgul 6  Hombres 7  Elfos 5  Enanos 7  Orcs 4  Hobbits 7  Mago 4   = 45

Gollum closes both lists and is not touched: its last byte is the base of the next list.

The three race changes were asked for in the singular, but Brujo and Hum sit in both tables — leaving the plural alone would have left the game saying "Formacion de 005 Hum". It was changed too, in the plural (Magos, Hombres), which is what that place wants: its neighbours are Enanos, Orcs and Hobbits. Elfos was already right. Race 7 of the plural table was written by the game in the singular (Brujo ), and that is respected: it reads Mago.

The list of 24 proper names, 0x6B46, separated by 0xB7 and copied up to that separator (0x6E23, 0x6F38). Brand III → Bardo III, nine letters for nine.

And the sheet's six adjectives, which are in no list at all. Each one is loaded by its own absolute ld hl — 0x704B, 0x7061, 0x7006, 0x6FEF, 0x701E and 0x7035 — so unlike everything above they can grow: move the string and change the pointer. Four of them do:

pointer	was	is	where it lives now
`0x7006`	Habil	Firme	`0x7DE6`, its own slot: five letters for five
`0x6FEF`	Valioso	Virtuoso	`0x6689`, in the dead beeper engine
`0x701E`	Duro	Valiente	`0x6692`
`0x7035`	Bravo	Fuerte	`0x669B`

The three that moved take 25 bytes and their three pointers six more. Enérgico and Decidido stay as they were, and so do the three abandoned strings at 0x7DF0-0x7DFF: nobody reads them any more, and a check makes sure they are left untouched.

The last line of the sheet was a template plus a word from the list at 0x7D6A, which is how it read Aliado a la Sociedad. Comunidad is one letter longer, so the whole list moved to 0x66A2 with each entry carrying the complete phrase, and the write starts at column 0 (0x7CEF) instead of column 10. That way the other three entries read exactly as they read before.

The font settles which letters are available: 0xC800 holds 128 characters of eight bytes, and everything from 0x21 to 0x7F is drawn (only 0x20, the space, is blank). Codes with bit 7 set are not letters — they are artwork from the 0x9E00 table — so there are no accents, and Nazgul keeps going without its circumflex.

6 · The map, repainted

The map is drawn with 128 tiles of 8 × 8 at 0x9E00, nine bytes each: eight of bitmap and a ZX Spectrum attribute behind. tools/lienzos.py exports the lot to a PNG of 128 × 64 at 1:1 — sixteen tiles per row — and reads it back; make parche compares the canvas against the cassette and turns every drawing that changed into an entry. 122 of the 128 changed.

The thing worth knowing is what colours can be asked for, because the attribute is the Spectrum's but the colour is the MSX's:

ATRIBUTO_A_COLOR (0x049F):  ld hl,004ceh   ; the table without bright
                            bit 6,a
                            jr z,+3
                            ld hl,004d6h   ; and the one with it
                            ...            ; ink -> high nibble, paper -> low

Two tables of eight bytes, read out of the tape:

	0	1	2	3	4	5	6	7
`0x04CE`, no bright	1	4	6	13	12	7	10	15
`0x04D6`, bright	1	5	9	13	3	7	11	15

Sixteen slots, twelve distinct MSX colours: nothing produces the medium red (8), the medium green (2) or the grey (14). And only four of the eight — blue, red, green and yellow — actually change with the bright bit, so the Spectrum's "both of the same brightness" rule only binds for those. The tool knows all of this: it draws the canvas in MSX colours, refuses a cell with three of them, and if an unreachable colour gets in it takes the nearest one and says so.

And one byte more, for the paper. UN_CARACTER_NORMAL (0x7616) paints every character of the font with one fixed attribute, the ld a,078h at 0x763E. Its operand — 0x763F — goes from 0x78 to 0x70: paper 6 with bright, which the 0x04D6 table sends to MSX colour 11, the same khaki the frames are painted in. It takes the spaces with it, which is what fills the inside of a signpost, so the box comes out khaki all through instead of leaving a halo behind each letter. It is global: menu, labels, sheet and battle.

What could not be done, and why, is in Findings.