Why does Apple use R8l for the byte registers instead of R8b?

Question

I saw this in Making Code 64-Bit Clean topic

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║ Register name       │ Descripti         


        

Answer 1

As from Ross Ridge's comment, that's because of the difference in AMD and Intel's register naming convention

From AMD64 Architecture Programmer’s Manual, Volume 1: Application Programming

3.1.2 64-Bit-Mode Registers

In 64-bit mode, eight new GPRs are added to the eight legacy GPRs, all 16 GPRs are 64 bits wide, and the low bytes of all registers are accessible. Figure 3-3 on page 27 shows the GPRs, flags register, and instruction-pointer register available in 64-bit mode. The GPRs include:

• Sixteen 8-bit low-byte registers (AL, BL, CL, DL, SIL, DIL, BPL, SPL, R8B, R9B, R10B, R11B, R12B, R13B, R14B, R15B).
• Four 8-bit high-byte registers (AH, BH, CH, DH), addressable only when no REX prefix is used.
• Sixteen 16-bit registers (AX, BX, CX, DX, DI, SI, BP, SP, R8W, R9W, R10W, R11W, R12W, R13W, R14W, R15W).
• Sixteen 32-bit registers (EAX, EBX, ECX, EDX, EDI, ESI, EBP, ESP, R8D, R9D, R10D, R11D, R12D, R13D, R14D, R15D).
• Sixteen 64-bit registers (RAX, RBX, RCX, RDX, RDI, RSI, RBP, RSP, R8, R9, R10, R11, R12, R13, R14, R15).

Compared to Intel® 64 and IA-32 Architectures - Software Developer’s Manual Volume 1: Basic Architecture

3.7.2.1 Register Operands in 64-Bit Mode

Register operands in 64-bit mode can be any of the following:

• 64-bit general-purpose registers (RAX, RBX, RCX, RDX, RSI, RDI, RSP, RBP, or R8-R15)
• 32-bit general-purpose registers (EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP, or R8D-R15D)
• 16-bit general-purpose registers (AX, BX, CX, DX, SI, DI, SP, BP, or R8W-R15W)
• 8-bit general-purpose registers: AL, BL, CL, DL, SIL, DIL, SPL, BPL, and R8L-R15L are available using REX prefixes; AL, BL, CL, DL, AH, BH, CH, DH are available without using REX prefixes.