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What are Symbolic Microinstructions?
The microinstructions can be determined by symbols. It is interpreted to its binary format with an assembler. The symbols should be represented for each field in the microinstruction. The users should be enabled to represent their symbolic addresses. Each line in an assembly language represents symbolic instruction. These instructions are divided into five fields such as label, micro-operations, CD, BR, and AD.
The fields that specify the following information are as follows −
- The label field may be empty or it may specify a symbolic address. A label is terminated with a colon (:).
- The micro-operations field consists of one, two, or three symbols, separated by commas. But each F field includes only a single symbol.
- The CD field has one of the letters U, I, S, or Z.
- The BR field contains one of the four symbols defined.
- The AD field specifies a value for the address field of the microinstruction in one of three possible ways −
- With a symbolic address, which must also appear as a label.
- With the symbol NEXT to designate the next address in sequence.
- When the BR field includes a RET or MAP symbol, the AD field is left null and is transformed to seven zeros by the assembler.
Fetch Routine
The control unit includes 128 words, each including 20 bits. The value of each bit should be specified to microprogram the control memory. Between the 128 words, the first 64 are composed for the routines of 16 instructions. The remaining 64 can be used for different goals. The best opening location for the fetch routine to start is the 64th address.
The microinstructions necessary for fetch routine are −
AR ← PC
DR ← M[AR], PC ← PC + 1
AR ← DR(0 − 10), CAR(2 − 5) ← DR(11 − 14), CAR(0,1,6) ← 0
The address of the instruction is transferred from PC to AR and the instruction is then read from memory into DR. Since no instruction register is available, the instruction code remains in DR. The address part is transferred to AR and then control is transferred to one of 16 routines by mapping the operation code part of the instruction from DR into CAR.
Microinstructions that are situated in addresses 64, 65, and 66 are important for the fetch routine. There are various symbolic language is as follows −
| ORG 64 | |
| FETCH: | PCTAR U JMP NEXT |
| READ, INCPC U JMP NEXT | |
| DRTAR U MAP |
The table shows the results of binary translation for the assembly language.
Binary Translation for Assembly Language
| Binary Address | F1 | F2 | F3 | CD | BR | AD |
|---|---|---|---|---|---|---|
| 1000000 | 110 | 000 | 000 | 00 | 00 | 1000001 |
| 1000001 | 000 | 100 | 101 | 00 | 00 | 1000010 |
| 1000010 | 101 | 101 | 000 | 00 | 11 | 0000000 |
Each microinstruction executes the internal register transfer operation displayed by the register transfer representation. The representation in symbols is important while writing microprograms in an assembly language format. The actual internal content which is saved in the control memory is in binary representation.
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