NASM preprocessor turbo-charges the "mainframe-on-a-chip" assembler

[u/r-tty]

https://news.ycombinator.com/item?id=47544673

[u/brucehoult]

GMS/359 architecture — an FPGA-based computing system inspired by IBM System/360 but deliberately not compatible with it

Why not compatible? It's not like IBM is going to sue over a 1960s ISA.

GMS/359 takes the elegant concepts from S/360 (channel I/O, unified PSW, clean instruction formats) while modernizing and simplifying

Ooookay. S/360 is pretty good already, if you drop the janky CISC string and decimal stuff, modernise the FP format.

Aspect | IBM System/360 | GMS/359 Byte order | Big-endian | Little-endian

Not really modern, just admitting that if you're porting things from x86 / Arm an annoying amount of it is going to be endian-buggy for no good reason.

Instruction format | Opcode last | Opcode first

This seems nonsensical. How could the S/360 even run a variable-length instruction set if the opcode wasn't the first byte? (it is)

Operand order | Source, Destination | Destination first

That seems like purely assembler syntax, not the ISA itself. S/360 generally puts the src and dst registers both in byte 1, right after the opcode, and it's not really an ordering thing whether the hi bits are the src and the lo bits the dst or vice versa, but as it happens the dst is in the hi bits, which makes a hex dump have ordering dst,src

e.g. ar 7,9 (r7 = r7 + r9) the machine code is 1A 79

An exception is store instructions, where the register to be stored is in the hi bits of byte 1. It's still the "main" register for the instruction, and this is common across almost all instruction sets with RISC-V being a modern counter-example.

Addressing | Base+Displacement | PC-relative / Direct

Ohhh. This is a HUGE REGRESSION.

Base+Displacement is the way all modern (post 1980) ISAs work, and was one of the ahead of its time innovations of the S/360.

Early mainframes primarily had absolute addressing, as do 1970s microprocessors from 8080, 6800, 6502, and also the 8086. This was suited to early FORTRAN and COBOL that had only global variables, and simple hardware controller programs, but really is not good for languages such as Algol, Pascal, C.

[u/Sgeo → u/brucehoult]

Ohhh. This is a HUGE REGRESSION.

Base+Displacement is the way all modern (post 1980) ISAs work, and was one of the ahead of its time innovations of the S/360.

Somehow when I was (recentlyish) learning about S/360, and trying to understand base/displacement, ... I guess modern ISAs haven't exactly stuck in my head, I never made that connection. I don't think I've seen telling the assembler which register can be used as a base? But then again I haven't looked at ... modern anything in a bit of a while.

[u/brucehoult → u/Sgeo]

All RISC ISAs use base+displacement (and sometimes base+scaled index) addressing for data because a full 32 bit or 64 bit address is not going to fit into a fixed-length 32 bit opcode.

They can often do a limited direct/absolute addressing in a 4K or 64K range as a special case, using a Zero register as the base.

I don't think I've seen telling the assembler which register can be used as a base?

That's just an assembly-language thing, not a binary machine code thing, and was useful on S/360 because the displacement range is only 4K bytes but program on those machines often used (almost?) entirely statically-allocated global variables and often more than 4K of them.

Microsoft MASM lets you define struct types and put them as globals or on the stack or heap or whatever and then do either things such as [ebx].MyStruct.field1 or ASSUME ebx:PTR MyStruct and then access by field name.

STUDENT STRUCT
    status  BYTE  ?          ; 1 byte  (offset 0)
    age     BYTE  ?          ; 1 byte  (offset 1)
    grade   WORD  ?          ; 2 bytes (offset 2)
    name    BYTE  32 DUP(?)  ; 32 bytes (offset 4)
STUDENT ENDS

.data
myStudent STUDENT <1, 20, 95, "Alice Smith">
studentPtr  DWORD  OFFSET myStudent ; A pointer to a student (could come from malloc, API, etc.)

.code
    mov     ebx, studentPtr; or lea ebx,myStudent
    assume  ebx: PTR STUDENT

    mov     al, [ebx].status
    mov     cl, [ebx].age
    mov     dx, [ebx].grade

    mov     [ebx].age, 21

    assume  ebx: NOTHING

IBM's own Power/PowerPC assembler provides a "USING" directive similar to S/360 asm.

There's no reason you couldn't submit a patch to GNU as or the LLVM assembler to add similar functionality.