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- .bp
- .AP "EM CODE TABLES"
- The following table is used by the assembler for EM machine
- language.
- It specifies the opcodes used for each instruction and
- how arguments are mapped to machine language arguments.
- The table is presented in three columns,
- each line in each column contains three or four fields.
- Each line describes a range of interpreter opcodes by
- specifying for which instruction the range is used, the type of the
- opcodes (mini, shortie, etc..) and range for the instruction
- argument.
- .QQ
- The first field on each line gives the EM instruction mnemonic,
- the second field gives some flags.
- If the opcodes are minis or shorties the third field specifies
- how many minis/shorties are used.
- The last field gives the number of the (first) interpreter
- opcode.
- .LP
- Flags :
- .IP ""
- Opcode type, only one of the following may be specified.
- .RS
- .IP \-
- opcode without argument
- .IP m
- mini
- .IP s
- shortie
- .IP 2
- opcode with 2-byte signed argument
- .IP 4
- opcode with 4-byte signed argument
- .IP 8
- opcode with 8-byte signed argument
- .IP u
- opcode with 2-byte unsigned argument
- .RE
- .IP ""
- Secondary (escaped) opcodes.
- .RS
- .IP e
- The opcode thus marked is in the secondary opcode group instead
- of the primary
- .RE
- .IP ""
- restrictions on arguments
- .RS
- .IP N
- Negative arguments only
- .IP P
- Positive and zero arguments only
- .RE
- .IP ""
- mapping of arguments
- .RS
- .IP w
- argument must be divisible by the wordsize and is divided by the
- wordsize before use as opcode argument.
- .IP o
- argument ( possibly after division ) must be >= 1 and is
- decremented before use as opcode argument
- .RE
- .LP
- If the opcode type is 2,4 or 8 the resulting argument is used as
- opcode argument (least significant byte first).
- If the opcode type is mini, the argument is added
- to the first opcode \- if in range \- .
- If the argument is negative, the absolute value minus one is
- used in the algorithm above.
- .br
- For shorties with positive arguments the first opcode is used
- for arguments in the range 0..255, the second for the range
- 256..511, etc..
- For shorties with negative arguments the first opcode is used
- for arguments in the range \-1..\-256, the second for the range
- \-257..\-512, etc..
- The byte following the opcode contains the least significant
- byte of the argument.
- First some examples of these specifications.
- .IP "aar mwPo 1 34"
- .br
- Indicates that opcode 34 is used as a mini for Positive
- instruction arguments only.
- The w and o indicate division and decrementing of the
- instruction argument.
- Because the resulting argument must be zero ( only opcode 34 may be used),
- this mini can only be used for instruction argument 2.
- Conclusion: opcode 34 is for "AAR 2".
- .IP "adp sP 1 41"
- .br
- Opcode 41 is used as shortie for ADP with arguments in the range
- 0..255.
- .IP "bra sN 2 60"
- .br
- Opcode 60 is used as shortie for BRA with arguments \-1..\-256,
- 61 is used for arguments \-257..\-512.
- .IP "zer e\- 145"
- .br
- Escaped opcode 145 is used for ZER.
- .LP
- The interpreter opcode table:
- .DS
- .so itables
- .DE
- .PP
- The table above results in the following dispatch tables.
- Dispatch tables are used by interpreters to jump to the
- routines implementing the EM instructions, indexed by the next opcode.
- Each line of the dispatch tables gives the routine names
- of eight consecutive opcodes, preceded by the first opcode number
- on that line.
- Routine names consist of an EM mnemonic followed by a suffix.
- The suffices show the encoding used for each opcode.
- .LP
- The following suffices exist:
- .TS
- tab(:);
- l l.
- .z:no arguments
- .l:16-bit argument
- .L:32-bit argument
- .u:16-bit unsigned argument
- .lw:16-bit argument divided by the wordsize
- .Lw:32-bit argument divided by the wordsize
- .p:positive 16-bit argument
- .P:positive 32-bit argument
- .pw:positive 16-bit argument divided by the wordsize
- .Pw:positive 32-bit argument divided by the wordsize
- .n:negative 16-bit argument
- .N:negative 32-bit argument
- .nw:negative 16-bit argument divided by the wordsize
- .Nw:negative 32-bit argument divided by the wordsize
- .s<num>:shortie with <num> as high order argument byte
- .w<num>:shortie with argument divided by the wordsize
- .<num>:mini with <num> as argument
- .<num>W:mini with <num>*wordsize as argument
- .TE
- .LP
- <num> is a possibly negative integer.
- .LP
- The dispatch table for the 256 primary opcodes:
- .sp 1
- .so dispat1
- .sp 2
- The list of secondary opcodes (escape1):
- .sp 1
- .so dispat2
- .sp 2
- Finally, the list of opcodes with four byte arguments (escape2).
- .sp 1
- .so dispat3
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