runtime: add a basic stdlib and crt0 file

examples/lib/crt0.s

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+// A minimal crt0.s that works along the stdlib.s file provided to give
+// some resemblance of a functioning compilation target :)
+//
+// Copyright (c) 2023 Anton Lydike
+// SPDX-License-Identifier: MIT
+
+.text
+
+.globl _start
+_start:
+        // TODO: read argc, argv from a0, a1
+        // maybe even find envptr?
+        jal main
+        jal exit

examples/lib/stdlib.s

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+// A very basic implementation of a stdlib.h but in assembly.
+// should(tm) work with riscemu.
+//
+// Copyright (c) 2023 Anton Lydike
+// SPDX-License-Identifier: MIT
+
+.data
+
+_rand_seed: 
+.word   0x76767676
+_atexit_calls:
+// leave room for 8 atexit handlers here for now
+.word   0x00, 0x00, 0x00, 0x00
+.word   0x00, 0x00, 0x00, 0x00
+_atexit_count:
+.word   0x00
+
+_malloc_base_ptr:
+// first word is a pointer to some space
+// second word is the offset inside that space
+// space is always MALLOC_PAGE_SIZE bytes
+.word   0x00, 0x00
+.equ    MALLOC_PAGE_SIZE 4069
+
+.text
+
+// malloc/free
+
+.globl malloc
+.globl free
+
+// malloc(size_t size)
+malloc:
+        // set aside size in s0
+        sw  s0, -4(sp)
+        mv  a0, s0
+        la  t0, _malloc_base_ptr
+        lw  t1, 0(t0)
+        beq t1, zero, _malloc_init
+_malloc_post_init:
+        // if we are here, we always have
+        // t0 = (&_malloc_base_ptr)
+        // t1 = *(&_malloc_base_ptr)
+        // new we load
+        // t2 = base_ptr_offset
+        lw  t2, 4(t0)
+        // add allocated size to offset
+        add t2, t2, s0
+        // check for overflow
+        li  t4, MALLOC_PAGE_SIZE
+        bge t2, t4, _malloc_fail
+        // save the new offset
+        sw  t2, 4(t0)
+        // calculate base_ptr + offset
+        add a0, t2, t1
+        // return that
+        lw  s0, -4(sp)
+        ret
+
+_malloc_init:
+        // call mmap2()
+        li  a0, 0       // addr = 0, let OS choose address
+        li  a1, 4096    // size
+        li  a2, 3       // PROT_READ   | PROT_WRITE
+        li  a3, 5       // MAP_PRIVATE | MAP_ANONYMOUS
+        li  a7, SCALL_MMAP2
+        ecall           // invoke syscall
+        // check for error code
+        li  t0, -1
+        beq a0, t0, _malloc_fail
+        // if succeeded, load &_malloc_base_ptr
+        la  t0, _malloc_base_ptr
+        // put value of _malloc_base_ptr into t1
+        mv  a0, t1
+        // save base ptr to _malloc_base_ptr
+        sw  t1, 0(t0)
+        // jump to post_init
+        j   _malloc_post_init
+_malloc_fail:
+        li a0, 0
+        ret
+
+// free is a nop, that's valid, but not very good^^
+free:
+        ret
+
+
+
+// exit, atexit
+.globl exit
+.globl atexit
+
+// we can happily use saved registers here because we don't care at all if we
+// destroy the calling registers. This is __noreturn__ anyways!
+// register layout:
+// s0 = &_atexit_count
+// s2 = &_atexit_calls
+// s1 = updated value of atexit
+exit:
+        la      s0, _atexit_count       // s0 = &_atexit_count
+        lw      s1, 0(s0)               // s1 = *(&_atexit_count)
+        // exit if no atexit() calls remain
+        beq     s1, zero, _exit
+        // decrement
+        addi    s1, s1, -4              // s1--
+        // save decremented value
+        sw      s1, 0(s0)               // _atexit_count = s1
+        li      s2, _atexit_calls
+        add     s1, s1, s2              // s1 = &_atexit_calls + (s1)
+        lw      s1, 0(s1)               // s1 = *s1
+        la      ra, exit                // set ra up to point to exit
+        jalr    zero, s1, 0             // jump to address in s1
+        // jalr will call the other function, which will then return back
+        // to the beginning of exit.
+_exit:
+        li      a0, 0
+        li      a7, 93
+        ecall
+
+// atexit a0 = funcptr
+atexit:
+        sw      t0, -4(sp)
+        sw      t2, -8(sp)
+        // load _atexit_count
+        la      t0, _atexit_count
+        lw      t2, 0(t0)
+        // if >= 8, fail
+        li      t1, 8
+        bge     t2, t1, _atexit_fail
+        // increment atexit_count by 4 (one word)
+        addi    t2, t2, 4
+        sw      t2, 0(t0)
+        // load address of _atexit_calls
+        la      t0, _atexit_calls
+        // add new _atexit_count to _atexit_calls
+        add     t0, t0, t2
+        sw      a0, -4(t0)
+        li      a0, 0
+        lw      t0, -4(sp)
+        lw      t2, -8(sp)
+        ret
+
+_atexit_fail:
+        li      a0, -1
+        lw      s0, -4(sp)
+        lw      s1, -8(sp)
+        ret
+
+
+
+        
+
+
+// rand, srand
+
+.globl  rand
+.globl  srand
+
+// simple xorshift rand implementation
+rand:
+        // load seed
+        la  t1, _rand_seed
+        lw  a0, 0(t1)
+        // three rounds of shifts:
+        sll a0, t0, 13          // x ^= x << 13;
+        srl a0, t0, 17          // x ^= x >> 17;
+        sll a0, t0, 5           // x ^= x << 5;
+	    sw  a0, 0(t1)
+	    ret
+
+srand:
+        la  t1, _rand_seed
+        sw  a0, 0(t1)
+        ret
+
+