Binary ALU instructions

Assembler input:

ADC|ADD|AND|BIC|EOR|ORR|RSB|RSC|SBC|SUB[S] <dest>,<source>,<shift>

Assembler output:

• High byte as standard
• Sixth nybble contains AL subinstruction code and S flag
• Bits 21-23 (3-bit cardinal) are AL subinstruction code
• Bit 20 is S flag
• Fifth nybble is first source register
• Fourth nybble is destination register
• Bottom three nybbles depend on base instruction grouping

If base instruction code is 0 or 1 then:

• Bit 4 determines whether shift factor is a register or a constant

  0

  Shift factor is a constant

  • Bits 7-11 (5-bit cardinal) are shift factor
  • Bits 5-6 are barrel shifter subinstruction code

  1

  Shift factor is a register

  • Third nybble is register containing shift factor
  • Bit 7 is 0 (indicating AL instruction, NOT multiply instruction)
  • Bits 5-6 are barrel shifter subinstruction code

• Low nybble is second source register

If base instruction code is 2 or 3 then:

• Bottom three nybbles are an immediate constant

The following subinstruction codes apply if the base instruction code is 0 or 2.

The following subinstruction codes apply if the base instruction code is 1 or 3.