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diff --git a/docs/exec-formats-and-tools.md b/docs/exec-formats-and-tools.md new file mode 100644 index 0000000..4ae43e4 --- /dev/null +++ b/docs/exec-formats-and-tools.md @@ -0,0 +1,242 @@ +QtMips - Supported Executable Formats and Toolchains +==================================================== + +The simulator accepts ELF statically linked executables +compiled for 32-bit big-endian MISP target. +Optimal is selection of some basic/initial mips +architecture and the use of plain mips-elf GCC toolchain. + +Then the next simple template can be used to compile +assembly source codes. + +''' +#define zero $0 +#define at $1 +#define v0 $2 +#define v1 $3 +#define a0 $4 +#define a1 $5 +#define a2 $6 +#define a3 $7 + +#define t0 $8 +#define t1 $9 +#define t2 $10 +#define t3 $11 +#define t4 $12 +#define t5 $13 +#define t6 $14 +#define t7 $15 +#define t8 $24 +#define t9 $25 +#define k0 $26 +#define k1 $27 + +#define s0 $16 +#define s1 $17 +#define s2 $18 +#define s3 $19 +#define s4 $20 +#define s5 $21 +#define s6 $22 +#define s7 $23 + +#define gp $28 +#define sp $29 +#define fp $30 +#define ra $31 + +.globl _start +.set noat +.set noreorder +.ent _start + +_start: + + // write the code there + +loop: break + beq zero, zero, _start + nop + +.end _start + +''' + + +The simulator recognizes 'break' instruction +compiled into the program. When 'break' instruction +is fetched, exception propagates through the pipeline. +The simulator waits till the pipeline is empty +and stops the continuous execution. + +PC is set to the next instruction in the program +flow. When a single step or continuous execution +is activated again then the program continues after the 'break' instruction. + +Another option is to set up "hardware" breakpoint for a given +instruction address. When the instruction from address marked +by breakpoint is fetched then the exception propagates through +the pipeline, all externally visible effects of instruction marked +by breakpoint are masked out in memory phase, all following +instructions in the pipeline are discarded and execution stops. +PC is set to the address of instruction causing the exception +or to the branch instruction address if the exception occurs in +delay slot. When the single step or continuous execution +is requested again then the "hardware" breakpoint exception +in the fetch stage is masked for the first executed instruction which. +But then CPU accepts breakpoint exceptions again. This is why it +is not a good idea to set up breakpoint to address of an instruction +in the delay slot. + +To compile simple assembly code programs invoke GCC with next options. +''' +mips-elf-gcc -ggdb -c program.S -o program.o +mips-elf-gcc -ggdb -nostdlib -nodefaultlibs -nostartfiles program.o -o program +''' + +Compile Executables with 'mips-linux-gnu' Toolchain +--------------------------------------------------- + +The Linux targetting toolchains use a MIPS O32 ABI calling +convention which allows building position-independent +binaries and overcome missing PC relative support +in the basic MIPS instruction set. The ABI uses strictly +convention equivalent to calling each function indirectly +through 't9' register ('jalr t9'). The known value pointing +to a start of called function allows computing Global Offset +Table (GOT) and global data pointers values from known offset +and 't9' register value. The startup code which conforms +these requirements to call 'main()' function looks like this: + + +''' +/* file crt0local.S - replacement of C library complete startup machinery */ +.globl main +.globl __start +.set noat +.set noreorder +.ent __start + +.text + +__start: + bal next + nop +next: .set noreorder + .cpload $31 + .set reorder + + addi a0, zero, 0 + addi a1, zero, 0 + jal main +quit: + addi a0, zero, 0 + addi v0, zero, 4001 /* SYS_exit */ + + syscall + +loop: break + beq zero, zero, loop + nop + +.end __start +''' + +The sequence of relative jump and link to next +instruction setups return address register 'ra' +to the value of the next instruction after 'bal', +and the delay slot. Actual translated code replaces +pseudo-instruction '.cpload' by sequence adding +offset into global data area from actual instruction +address and stores result in the 'gp' + + +''' +00400500 <__start>: + 400500: 04110001 bal 400508 <next> + 400504: 00000000 nop +00400508 <next>: + .cpload $31 + 400508: 3c1c000a lui gp,0xa + 40050c: 279c6238 addiu gp,gp,25144 + 400510: 039fe021 addu gp,gp,ra + 400514: 20040000 addi a0,zero,0 + 400518: 20050000 addi a1,zero,0 + 40051c: 8f998018 lw t9,-32744(gp) + 400520: 0411ffc6 bal 40043c <main> + 400524: 00000000 nop +00400528 <quit>: + 400528: 20040000 addi a0,zero,0 + 40052c: 20020fa1 addi v0,zero,4001 + 400530: 0000000c syscall +00400534 <loop>: + 400534: 0000000d break + 400538: 1000fffe b 400534 <loop> + 40053c: 00000000 nop +''' + +It can be seen that assembler not only expands '.cpload' +but even marks 'jalr t9' instruction as a target for +link time optimization. Because 'main' function is near +(offset less than 128 kB) to the startup code, the 'jal' +instruction is replaced by 'bal main' during linking phase. + + +Then simple main function which outputs string to the +serial port provided by QtMisp emulator can be implemented +as: + +''' +/* serial-port-test.c */ +#include <stdint.h> + +#define SERIAL_PORT_BASE 0xffffc000 +#define SERP_ST_REG_o 0x00 +#define SERP_ST_REG_TX_BUSY_m 0x1 +#define SERP_TX_REG_o 0x04 + +static inline void serp_write_reg(uint32_t base, uint32_t reg, uint32_t val) +{ + *(volatile uint32_t *)(base + reg) = val; +} +static inline uint32_t serp_read_reg(uint32_t base, uint32_t reg) +{ + return *(volatile uint32_t *)(base + reg); +} + +void serp_tx_byte(int data) +{ + while (serp_read_reg(SERIAL_PORT_BASE, SERP_ST_REG_o) & + SERP_ST_REG_TX_BUSY_m); + serp_write_reg(SERIAL_PORT_BASE, SERP_TX_REG_o, data); +} + +int main(void) +{ + const char *text = "Hello world.\nI am alive.\n"; + const char *s; + + for (s = text; *s; s++) + serp_tx_byte(*s); + + return 0; +} +''' + +Compilation: + + +''' +mips-linux-gnu-gcc -ggdb -fno-lto -c crt0local.S -o crt0local.o +mips-linux-gnu-gcc -ggdb -O3 -fno-lto -c serial-port-test.c -o serial-port-test.o +mips-linux-gnu-gcc -ggdb -nostartfiles -static -fno-lto crt0local.o serial-port-test.o -o serial-port-test +''' + +The simulator implements 'rdhwr $rd, userlocal' instrcution +and allows code compiled agains musl C library to start as well. + +''' +mips-linux-gnu-gcc -ggdb -O1 -march=mips2 -static -specs=/opt/musl/mips-linux-gnu/lib/musl-gcc.specs -c program.c -o program.o +mips-linux-gnu-gcc -ggdb -march=mips2 -static -specs=/opt/musl/mips-linux-gnu/lib/musl-gcc.specs programo -o program +''' |