104 lines
4.2 KiB
C
104 lines
4.2 KiB
C
// assembly.h
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// These are functions useful for translating assembly mnemonics to hex and vice-versa for the SplitBit CPU.
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// Written by Anachronaut
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// 10/18/2024
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#include <stdint.h>
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#ifndef ASSEMBLY_H
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#define ASSEMBLY_H
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// ---- The SplitBit binary format ----
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//
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// A binary starts with a file header, then the Program Segment, then the Data
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// Segment. All multi byte numbers are stored most significant byte first.
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//
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// Offset Size Field
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// 0 4 "SPBT", so a file that is not a SplitBit binary is spotted at once
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// 4 1 Format version
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// 5 4 Required feature flags
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// 9 3 "PRG"
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// 12 2 Program Segment length
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// 14 N Program Segment
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// .. 3 "DAT"
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// .. 2 Data Segment length
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// .. M Data Segment
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// .. 3 "VEC", optional
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// .. 2 Vector Segment length, in bytes
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// .. K Vector Segment, four bytes per entry
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//
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// The Vector Segment is optional and comes last, so a binary written before it existed
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// simply ends after its Data Segment and still loads. Each entry is two bytes saying
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// where in Program Memory the vector sits, then two bytes saying where its handler is,
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// most significant byte first. It is a list rather than an image of the table, so a
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// program with three handlers costs twelve bytes instead of a padded kilobyte.
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//
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// The feature flags are how a binary says it needs something the base machine does
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// not provide, so that an emulator which cannot provide it refuses to run the binary
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// rather than quietly doing the wrong thing. No features are defined yet; the field
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// is here so that adding one later does not need another format version.
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// ---- The vector table ----
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//
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// The top kilobyte of Program Memory is reserved for vectors. Both tools have to
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// agree on where it begins: the CPU starts execution through it, and the assembler
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// has to refuse program text that would run into it.
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//
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// Entries are two bytes each, most significant byte first, the same order the branch
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// instructions and this file format already use.
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//
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// 0xFC00 Software vectors 0 to 255
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// 0xFE00 Hardware vectors 0 to 255, one for each I/O port
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//
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// Software vectors 0 and 1 are start addresses rather than handlers. Vector 0 is
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// where the machine begins at power on and vector 1 is a warm restart, so a zero in
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// either of them is not "nothing installed" but the address 0x0000, which is where a
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// program carrying no vector table of its own begins. A zero in any other entry does
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// mean no handler is installed, and dispatching through one is a fault.
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#define SOFTWARE_VECTOR_BASE 0xFC00
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#define HARDWARE_VECTOR_BASE 0xFE00
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#define VECTOR_ENTRY_BYTES 2
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#define VECTOR_BOOT 0
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#define VECTOR_SOFT_RESET 1
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#define VECTOR_INVALID_OPCODE 2
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// Vectors 3 to 15 are held back for faults that do not exist yet, so that each cause
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// can have an entry of its own rather than sharing one and needing a cause register to
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// tell them apart. Everything from 16 up belongs to programs.
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#define VECTOR_FIRST_FREE 16
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// The first address the vector table occupies, and so the first address that program
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// text may not use.
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#define PROGRAM_TEXT_LIMIT SOFTWARE_VECTOR_BASE
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#define SPLITBIT_MAGIC "SPBT"
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#define SPLITBIT_MAGIC_LENGTH 4
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#define SPLITBIT_FORMAT_VERSION 1
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#define SPLITBIT_FLAGS_LENGTH 4
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#define SEGMENT_MARKER_LENGTH 3
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#define SEGMENT_LENGTH_BYTES 2
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// Where a vector sits, and where its handler is.
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#define VECTOR_ENTRY_FILE_BYTES 4
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// Everything the format costs a file, on top of the two segments themselves.
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#define SPLITBIT_HEADER_BYTES (SPLITBIT_MAGIC_LENGTH + 1 + SPLITBIT_FLAGS_LENGTH \
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+ 2 * (SEGMENT_MARKER_LENGTH + SEGMENT_LENGTH_BYTES))
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// Features this build of the emulator can provide. A binary asking for anything
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// outside this set is refused.
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#define SPLITBIT_FEATURES_SUPPORTED 0x00000000u
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// Features the assembler currently needs to ask for. Nothing, so far.
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#define SPLITBIT_FEATURES_REQUIRED 0x00000000u
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const char* getMnemonic(uint8_t opcode);
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uint8_t getOpcode(char* mnemonic);
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// How many Data Pointer selector bytes follow the given opcode. Never more than two.
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#define MAX_DATA_POINTER_OPERANDS 2
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int dataPointerOperands(uint8_t opcode);
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#endif // CPU_H
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