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You are here: OSU8 Microprocessor CPU Programming Assembler

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OSU8 Microprocessor Overview CPU Programming Instruction Set Assembler Software Simulator Hardware Info Schematic Implementation Download Files

Drafting The Assembly Language

It was also obvious that the assembly must have built in macros for some common cases, such as loading registers with immediate data, since the instruction set only supports loading 4 bits at a time.

Writing the Assembler

I should add more detail about the internals of the assembler... maybe at the bottom of this page...

Example Code

Example Source Code

Resulting List Output

; this uses some of the nifty macros, ; it fills all of the machine's memory, ; except for it's own space, with values ; between "minval" and "maxval" .equ minval, 'a' .equ maxval, 'z' .org 0 begin: move #stack -> p1 move p1 -> sp move #begin -> p1 restart:move #end -> p2 inc p2 loop0: move #minval -> a loop: store a -> @p2 inc p2 jp begin cje a, #maxval, loop0 inc a setb c jc loop stack: nop ;some room for the stack. nop ;The stack isn't explicitly nop ;used, but some macros nop ;use the stack end: nop

0000: C7 MOVE #7 -> P1Lb 0001: D2 MOVE #2 -> P1Lt 0002: E0 MOVE #0 -> P1Hb 0003: F0 MOVE #0 -> P1Ht 0004: 77 MOVE P1 -> SP 0005: C0 MOVE #0 -> P1Lb 0006: D0 MOVE #0 -> P1Lt 0007: E0 MOVE #0 -> P1Hb 0008: F0 MOVE #0 -> P1Ht 0009: 46 PUSH P1H 000A: 44 PUSH P1L 000B: CB MOVE #B -> P1Lb 000C: D2 MOVE #2 -> P1Lt 000D: E0 MOVE #0 -> P1Hb 000E: F0 MOVE #0 -> P1Ht 000F: 75 MOVE P1 -> P2 0010: 45 POP P1L 0011: 47 POP P1H 0012: 71 INC P2 0013: 6E NOP 0014: 81 MOVE #1 -> Ab 0015: 96 MOVE #6 -> At 0016: 6E NOP 0017: 6E NOP 0018: 59 STORE A -> @P2 0019: 71 INC P2 001A: 64 MOVE P -> C 001B: 26 JCU 0000 001C: 42 PUSH B 001D: AA MOVE #A -> Bb 001E: B7 MOVE #7 -> Bt 001F: 65 CLR C 0020: 15 SUB A-B -> B 0021: 43 POP B 0022: 63 MOVE Z -> C 0023: 23 JCU 0014 0024: 0C INC A 0025: 66 SET C 0026: 22 JCU 0018 0027: 6E NOP 0028: 6E NOP 0029: 6E NOP 002A: 6E NOP 002B: 6E NOP

Here's a few details to look for in the above code:

• Macros to load immediate data into an entire register, such as MOVE #stack -> p1 are actually turned into two or four instructions.
• Labels loop0 and loop are targets of branch instructions. The assembler automatically inserted NOP instructions at 0013, 0016 and 0017, so that these labels end up aligned at valid branch locations. This is due to the OSU8's opcodes using only 4 bit for a relative offset, at 32 bit aligned boundries.
• The OSU8 instruction set only provides conditional jump on carry. The assembler macros allow conditional jumps on any bit, by moving the bit into the carry.
• The CJE macro does a conditional jump if the two operands are equal. The assembler inserts 8 instructions to accomplish this, including a push and pop of the B register, which is used to hold the constant.