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--- sd:isa [2026/09/08 03:51] – external edit 127.0.0.1
+++ sd:isa [2026/09/08 04:07] (current) – external edit 127.0.0.1
@@ Line 180: / Line 180: @@
 Instructions added to remove work from a specific hot path rather than to add expressiveness. Each one has a primary consumer noted below.
-Canidates for inclusion: LEA, fused CMP-Bcc and CMP-Jcc, conditional move, LD_IDX16
+Canidates for inclusion: LEA, fused CMP-Bcc and CMP-Jcc, conditional move, LD_IDX16, [[#ptrace|PTRACE]] (proposed, see below)
 | #   | hex  | Mnemonic | Example          | Description                                  | Consumer | Flags |
@@ Line 629: / Line 629: @@
 **Measured.** 7.2% off the cost of a turn in rogueima -- more than MAC, because
 the branches it removes were unpredictable ones inside the hottest loop.
+<wrap #ptrace />
+**''PTRACE'' -- PROPOSED, NOT IMPLEMENTED**\\
+//No opcode assigned. This is a specification for review, not a description of
+the machine.//
+Walk a line through a tile array, stopping at the first tile that blocks, and
+optionally marking every tile visited on the way. A PPU instruction in the same
+family as ''PLINE'', and taking its parameters in registers the same way.
+^ ^ ^
+| **in** | ''ELM'' = tile array base |
+| ::: | ''X'',''Y'' = start;  ''I'',''J'' = target |
+| ::: | ''KL'',''KH'' = map width, height (for bounds) |
+| ::: | ''BL'' = bytes per tile;  ''BH'' = offset of the flags byte within a tile |
+| ::: | ''CL'' = block mask;  ''CH'' = set mask |
+| **out** | ''X'',''Y'' = where it stopped |
+| ::: | ''AL'' = tiles visited |
+| ::: | ''Z'' set if it stopped on a blocker, clear if it reached the target |
+At every tile along the line it ORs ''CH'' into the flags byte; if
+''flags & CL'' is non-zero it stops, having marked that tile. It also stops at
+the edge of the array. A ''CH'' of zero traces without marking, which is what a
+projectile or a "can this monster see me" test wants.
+<codify armasm>
+    ; one line-of-sight ray, marking SEEN|VISIBLE and stopping at anything opaque
+    LDELM [@level_tiles]
+    LDB [@level_dim]        ; KL, KH = width, height
+    MOV K, B
+    LDBL #2                 ; two bytes per tile
+    LDBH #1                 ; the flags are the second of them
+    LDCL @TF_OPAQUE         ; what stops the ray
+    LDCH $0C                ; TF_SEEN | TF_VISIBLE
+    LDXL [@PX]
+    LDYL [@PY]
+    LDI [@target_x]
+    LDJ [@target_y]
+    PTRACE
+</codify>
+**Why.** Rogueima's line of sight is 68 rays a turn, and the ray loop is about
+,000 of the 62,577 instructions a turn costs -- 48 ms at 1.3 MIPS. Each ray
+is roughly 540 instructions of Bresenham stepping and per-tile testing. As one
+instruction, a ray becomes "set ''I'',''J''; ''PTRACE''": three instructions
+instead of 540, and a turn drops to roughly 15 ms.
+**Where else.** The masks are what make it general. Projectile and thrown-object
+paths, wand and breath-weapon beams, monster targeting, swept collision in a
+tile platformer, and post-pathfinding line smoothing ("can I walk straight from
+here to there") are all this instruction with different masks and usually a
+''CH'' of zero.
+The application that makes it a platform capability rather than one game's
+helper is a **Wolfenstein-style raycaster**: one ray per screen column through a
+tile map, 320 rays a frame at 60 Hz. In software that is several million
+instructions a second, more than this machine has, so the genre is off the
+table. As an instruction it is about 19,000 a second plus the column drawing.
+**A design fork worth settling first.** Two different things wear similar
+clothes here:
+  * **Bresenham** visits the tiles a line passes through. Symmetric, integer,
+    and what line of sight, projectiles, targeting and collision all want.
+  * **DDA** steps to each grid //boundary// and carries the exact crossing
+    distance. That is what a perspective raycaster needs -- Wolfenstein used it
+    because Bresenham's nearest-tile approximation gives the wrong distance, and
+    a wrong distance is a wrong wall height and a fisheye.
+''PTRACE'' as specified above is the Bresenham walk. A perspective raycaster
+wants a sibling -- ''PCAST''? -- returning a fractional distance and which side
+of the tile was struck, for texture mapping. Trying to make one instruction do
+both would make both worse.
+The name deliberately says the job rather than the algorithm, so that the pair
+reads ''PTRACE''/''PCAST'' rather than ''PBRES''/''PDDA''.
+**Open questions.**
+  - Six registers in is a lot. Is a descriptor block at ''[FLX]'', the way
+    ''LSTEPM'' does it, a better fit for the array parameters, which are
+    level-scoped rather than per-call?
+  - Should it also report the tile it stopped ON versus the last clear tile
+    before it? Line of sight wants the blocker marked; a projectile wants the
+    square in front of it.
+  - Should a zero-length line (start == target) mark the start square?
+**Precedent.** No CPU precedent that I know of, but a strong coprocessor one:
+this is what blitters did. The Amiga blitter (1985) had a hardware Bresenham
+line mode, and the TMS34010 (1986) was a graphics processor with ''LINE'' as an
+instruction. It belongs to that tradition, which is where the rest of the PPU
+already lives.