Intel i960 Subroutines

Mark Smotherman. Last updated September 2002.

Introduction

The i960 is a 32-bit RISC architecture introduced in 1989. Features include:

• 16 global registers, and 16 local registers visible at one time
• parameters are passed through the global registers
• call and return instructions automatically save and restore fixed-sized windows (in blocks of 16 registers)
• branch and link instruction can be used to call a leaf procedure

Register conventions

        g0-g3    - parameters 0-3  /  return words 0-3
        g4-g7    - parameters 4-7  /  temporaries
        g8-g12   - preserved across call if not used for parameters
        g13      - structure return pointer
        g14      - argument block pointer  /  leaf return address (hardware)
        fp (g15) - frame pointer (hardware, aligned on 16-byte boundaries)

        pfp (r0) - previous frame pointer (hardware)
        sp  (r1) - stack pointer (hardware)
        rip (r2) - return instruction pointer (hardware)
        r3-r15   - available as locals for subroutine

Subroutine call instructions

        bal  target     - g14 <- return address;
                          PC <- target

        bx   (register) - PC <- register

        call target     - rip <- return address;
                          save r0-r15 to register cache (frame spill to
                            memory is handled by the hardware);
                          pfp <- fp;
                          fp <- sp {forced alignment on 16-byte boundary};
                          sp <- sp + 64;
                          PC <- target

        ret             - fp <- pfp;
                          restore r0-r15 from register cache (frame fill
                            from memory is handled by the hardware);
                          PC <- rip

Calling program structure

           ...

           ... place parameters in g0-g7 ...

           call subr

           ...

Subroutine structure

        subr:
           lda  N(sp),sp                ! adjust the stack pointer to
                                        !   allocate memory for locals
           movq g0,r4                   ! save input parameters in g0-g3
                                        !   into r4-r7

           ... body of subroutine ...

           ret

Leaf subroutine structure callable by either bal or call

        subr:                            ! call entry
           lda  subr_ret,g14             ! set up "return"
        _subr:                           ! bal entry
           mov  g14,g7                   ! move "return address" to g7

           ... body of subroutine ...

           mov  0,g14
           bx   (g7)                     ! branch either to return address
        subr_ret:                        !   directly or to subr_ret
           ret

Other resources

• i960 CA/CF Microprocessor User's Manual

• i960 Compiler User's Guide

[History of subroutines page] [Mark's homepage] [CPSC homepage] [Clemson Univ. homepage]

mark@cs.clemson.edu