c - 在 GBA tile 模式下将单字节写入 VRAM 的意外结果，值也写入下一个或前一个字节

Question

我正在尝试使用 GBA 拼贴模式从头开始显示具有单一彩色像素的单个拼贴。

它主要工作，但不是设置单个像素，而是在我打算写入的 16 位对齐位置为两个字节设置相同的颜色:

通过 mgba 模拟器运行时的结果图像: Screen

模拟器允许检查磁贴、内存和寄存器。
Tilesmap , 显示在图 block 上设置的两个像素。
VRAM ，从图 block 地址开始，显示确实有两个 8bpp 像素集，引用调色板中索引 42 处的颜色。
Display Control Register没有意外，在启用背景 0 的第一个平铺模式下运行。
Background 0 Control register , 在 8bpp 模式下。

主要内容:

#include "screen.h"

void do_it() {
  // [tile_index][row][column] = palette index
  VRAM.tilesets[3].d_tiles[37][0][0] = 42;
}


int main() {
  // [tilemap][row][column] = tile_index
  VRAM.tilemaps[0][0][0] = 37;
  do_it();
  PALETTE.backgrounds[42] = rgb(0b1111, 0b11111, 0);
  BG0CNT = BGCNT_SIZE_32X32 | BGCNT_COLOR_8BPP | (0 << BGCNT_TILEMAP_INDEX) | (3 << BGCNT_TILESET_INDEX);
  DISPCNT = DISPCNT_MODE_TILE_0 | DISPCNT_BACKGROUND_0;
  while (1) {};
}

screen.h 的有用部分:

typedef unsigned char tile_4bpp[8][4];
typedef unsigned char tile_8bpp[8][8];

typedef union {
  tile_4bpp s_tiles[512];
  tile_8bpp d_tiles[256];
} char_block;

extern union {
  char_block tilesets[4];
  screen_block tilemaps[32];
  color bitmap[HEIGHT][WIDTH];
} VRAM;

所以在函数 do_it 中，我设置了一个单字节值，只在硬编码位置设置一次。
然而在内存中，它将值设置为给定地址和完成 2 字节对齐的地址:
VRAM.tilesets[3].d_tiles[37][0][0] = 42; 和 VRAM.tilesets[3].d_tiles[37][0][1] = 42 ; 有相同的结果。

我认为我可能在我的结构/union/类型定义中做了一些糟糕的事情，但程序集看起来不错:

Disassembly of section .text:

080000e8 <do_it>:
 80000e8:   4b02        ldr r3, [pc, #8]    ; (80000f4 <do_it+0xc>)
 80000ea:   4a03        ldr r2, [pc, #12]   ; (80000f8 <do_it+0x10>)
 80000ec:   212a        movs    r1, #42 ; 0x2a
 80000ee:   5499        strb    r1, [r3, r2]
 80000f0:   46c0        nop         ; (mov r8, r8)
 80000f2:   4770        bx  lr
 80000f4:   06000000    streq   r0, [r0], -r0
 80000f8:   0000c940    andeq   ip, r0, r0, asr #18

080000fc <main>:
 80000fc:   b510        push    {r4, lr}
 80000fe:   4b09        ldr r3, [pc, #36]   ; (8000124 <main+0x28>)
 8000100:   2225        movs    r2, #37 ; 0x25
 8000102:   801a        strh    r2, [r3, #0]
 8000104:   f7ff fff0   bl  80000e8 <do_it>
 8000108:   4b07        ldr r3, [pc, #28]   ; (8000128 <main+0x2c>)
 800010a:   2254        movs    r2, #84 ; 0x54
 800010c:   4907        ldr r1, [pc, #28]   ; (800012c <main+0x30>)
 800010e:   5299        strh    r1, [r3, r2]
 8000110:   4b07        ldr r3, [pc, #28]   ; (8000130 <main+0x34>)
 8000112:   228c        movs    r2, #140    ; 0x8c
 8000114:   801a        strh    r2, [r3, #0]
 8000116:   4b07        ldr r3, [pc, #28]   ; (8000134 <main+0x38>)
 8000118:   2280        movs    r2, #128    ; 0x80
 800011a:   0052        lsls    r2, r2, #1
 800011c:   801a        strh    r2, [r3, #0]
 800011e:   46c0        nop         ; (mov r8, r8)
 8000120:   e7fd        b.n 800011e <main+0x22>
 8000122:   46c0        nop         ; (mov r8, r8)
 8000124:   06000000    streq   r0, [r0], -r0
 8000128:   05000000    streq   r0, [r0, #-0]
 800012c:   000003ef    andeq   r0, r0, pc, ror #7
 8000130:   04000008    streq   r0, [r0], #-8
 8000134:   04000000    streq   r0, [r0], #-0

我不精通 arm 汇编，但 do_it 中的 strb 似乎符合矫揉造作，它应该根据 arm doc 对单个字节进行操作.

更多信息:

• 它发生在多个模拟器上:我通常使用 mgba，它也发生在 vbam 上
• 其他 rom 在 mgba 上按预期工作
• 使用 arm-none-eabi-gcc 工具链 v 12.1.0 构建
• 没有任何优化标志
• 使用 -mthumb -mthumb-interwork -mcpu=arm7tdmi -fomit-frame-pointer -ffast-math -fno-strict-aliasing
• 4bpp 模式也会发生这种情况，设置四个像素而不是预期的两个
• 代码直接从 gamepak1 内存运行
• 当使用不同的 tilesets/tilemaps 位置时会发生这种情况
• 在位图模式下运行时不会发生:

这有效并在屏幕左上角显示一个绿色像素:

int main() {
  VRAM.bitmap[0][0] = rgb(0b1111, 0b11111, 0);
  DISPCNT = DISPCNT_MODE_3 | DISPCNT_BACKGROUND_2;
  while (1) {};
}

可以在此处找到完整的代码库:github

知道是什么原因造成的吗？

Answer 1

内存映射:

MEMORY
{
   ewram : ORIGIN = 0x02000000, LENGTH = 256K
}
SECTIONS
{
   .text : { *(.text*) } > ewram
}

启动.s

.cpu arm7tdmi
.code 32

.globl _start
_start:
    ldr sp,=0x03008000
    bl notmain
hang:
    b hang

.globl PUT16
PUT16:
    strh r1,[r0]
    bx lr

.globl GET16
GET16:
    ldrh r0,[r0]
    bx lr

.globl PUT32
PUT32:
    str r1,[r0]
    bx lr

.globl GET32
GET32:
    ldr r0,[r0]
    bx lr

notmain.c

extern void PUT32 ( unsigned int, unsigned int );
extern unsigned int GET32 ( unsigned int );
extern void PUT16 ( unsigned int, unsigned int );
extern unsigned int GET16 ( unsigned int );

#define DISPCNT 0x04000000
#define BG0CNT  0x04000008

#define PMEM 0x05000000
#define TMEM 0x06000000
#define VMEM 0x06008000

void notmain ( void )
{
    unsigned int ra;
    unsigned int rb;

    //display control,
    //mode 0
    //enable BG0
    PUT16(DISPCNT,0x0100);
    //BG0 control
    //256 color palette
    //tiles defined at 0x60000000
    //screen at 0x60008000
    PUT16( BG0CNT,0x1080);
    //setup the first 8 colors
    PUT16(PMEM+0x0,0x0000); //BLACK
    PUT16(PMEM+0x2,0x001F); //RED
    PUT16(PMEM+0x4,0x03E0); //GREEN
    PUT16(PMEM+0x6,0x03FF); //GREEN+RED
    PUT16(PMEM+0x8,0x7C00); //BLUE
    PUT16(PMEM+0xA,0x7C1F); //BLUE+RED
    PUT16(PMEM+0xC,0x7FE0); //BLUE+GREEN
    PUT16(PMEM+0xE,0x7FFF); //BLUE+GREEN+RED (WHITE)
    //lets make a few tiles 64 bytes per tile.
    ra=TMEM;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;

    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000000); ra+=4;
    PUT32(ra,0x00000000); ra+=4; PUT32(ra,0x00000002); ra+=4;

    //make screen black/clear screen
    ra=VMEM;
    for(rb=0;rb<(32*20);rb++)
    {
        PUT16(ra,0x0000);
        ra+=2;
    }
    //put some tiles on the screen
    PUT16(VMEM,1);
}

构建:

arm-none-eabi-as --warn --fatal-warnings  startup.s -o startup.o
arm-none-eabi-gcc -c -mcpu=arm7tdmi -Wall -O2 -ffreestanding  notmain.c -o notmain.o
arm-none-eabi-ld -nostdlib -nostartfiles -T memmap startup.o notmain.o -o notmain.elf
arm-none-eabi-objdump -D notmain.elf > notmain.list
arm-none-eabi-objcopy notmain.elf -O binary notmain.mb

运行:

vba notmain.mb

我运行的是 linux，所以我没有 vba-m，但我有 vba。这很好用。您可以用 copyjump 包装它或将其构建为 gba rom，您今天可以购买的卡带不关心 gba 文件中的标题标题，因为卡带固件是实际的 gba rom。 mgba 似乎想要正确的 header ，看起来您知道该怎么做。

您的关键问题是 VRAM 只能通过 16 位或 32 位传输(str 或 strh)而不是 8 位(strb)正确访问。它会弄乱那个半字中的另一个像素。

EWRAM  256Kbytes, can be written as bytes, halfwords, and words
IWRAM  32Kbytes, can be written as bytes, halfwords, and words
PRAM   512 halfwords, can be written as halfwords or words (not bytes)
VRAM   98Kbytes, can be written as halfwords or words (not bytes)