diff --git a/twasm/README.md b/twasm/README.md
index 2921ffa..77905a6 100644
--- a/twasm/README.md
+++ b/twasm/README.md
@@ -328,14 +328,14 @@ supported tokens are listed below
 | r13d  | 0x001D | unimplemented |
 | r14d  | 0x001E | unimplemented |
 | r15d  | 0x001F | unimplemented |
-| ax    | 0x0020 | unimplemented |
-| bx    | 0x0021 | unimplemented |
-| cx    | 0x0022 | unimplemented |
-| dx    | 0x0023 | unimplemented |
-| si    | 0x0024 | unimplemented |
-| di    | 0x0025 | unimplemented |
-| sp    | 0x0026 | unimplemented |
-| bp    | 0x0027 | unimplemented |
+| ax    | 0x0020 | |
+| bx    | 0x0021 | |
+| cx    | 0x0022 | |
+| dx    | 0x0023 | |
+| si    | 0x0024 | |
+| di    | 0x0025 | |
+| sp    | 0x0026 | |
+| bp    | 0x0027 | |
 | r8w   | 0x0028 | unimplemented |
 | r9w   | 0x0029 | unimplemented |
 | r10w  | 0x002A | unimplemented |
@@ -344,14 +344,14 @@ supported tokens are listed below
 | r13w  | 0x002D | unimplemented |
 | r14w  | 0x002E | unimplemented |
 | r15w  | 0x002F | unimplemented |
-| al    | 0x0030 | unimplemented |
-| bl    | 0x0031 | unimplemented |
-| cl    | 0x0032 | unimplemented |
-| dl    | 0x0033 | unimplemented |
-| sil   | 0x0034 | unimplemented |
-| dil   | 0x0035 | unimplemented |
-| spl   | 0x0036 | unimplemented |
-| bpl   | 0x0037 | unimplemented |
+| al    | 0x0030 | |
+| bl    | 0x0031 | |
+| cl    | 0x0032 | |
+| dl    | 0x0033 | |
+| sil   | 0x0034 | |
+| dil   | 0x0035 | |
+| spl   | 0x0036 | |
+| bpl   | 0x0037 | |
 | r8b   | 0x0038 | unimplemented |
 | r9b   | 0x0039 | unimplemented |
 | r10b  | 0x003A | unimplemented |
diff --git a/twasm/asm/main.asm b/twasm/asm/main.asm
index 9c052e1..0383da2 100644
--- a/twasm/asm/main.asm
+++ b/twasm/asm/main.asm
@@ -177,6 +177,32 @@ assemble:
     mov rsi, .msg_operator_1_register
     call print.debug
 
+    ; di = token table entry
+    call get_tte_typed_metadata
+    ; al = register typed metadata
+
+    and al, 11b ; al = register width
+
+    cmp al, 00b          ; 8 bit
+    je .unexpected_token ; TODO handle 8 bit opcodes
+
+    cmp al, 10b                        ; 32 bit
+    je .operator_1_register_no_prefix ; default register length; no prefix
+
+    cmp al, 01b ; 16 bit
+    je .operator_1_register_16
+
+    cmp al, 11b ; 64 bit
+    je .operator_1_register_64
+  .operator_1_register_16:
+    mov al, 0x66
+    call .push_byte
+    jmp .operator_1_register_no_prefix
+  .operator_1_register_64:
+    mov al, 0x48
+    call .push_byte
+    jmp .operator_1_register_no_prefix
+  .operator_1_register_no_prefix:
     mov si, di     ; si = `R/M` tte
     and edx, 0xFF
     or edx, 0xFE00  ; pass di as direct value
@@ -232,7 +258,24 @@ assemble:
     call .next_token
     jge .break
     call .get_next_tte
+    ; di = tte
+    call get_tte_typed_metadata
 
+    and al, 11b ; al = register width
+
+    cmp al, 10b ; 32 bit
+    je .operator_2_memory_32
+
+    cmp al, 11b ; 64 bit
+    je .operator_2_memory_continue
+
+    ; other cases: 16 bit, 8 bit both are not valid for addressing
+    jmp .size_mismatch
+  .operator_2_memory_32:
+    mov al, 0x67
+    call .push_byte
+    jmp .operator_2_memory_continue
+  .operator_2_memory_continue:
     mov si, di ; si = dst register tte
 
     call .next_token
@@ -253,13 +296,39 @@ assemble:
     je .operator_2_memory_register ; if so, handle
 
     jmp .unexpected_token
-
   .operator_2_memory_register:
     push rsi
     mov rsi, .msg_operator_2_memory_register
     call print.debug
     pop rsi ; si = r/m
 
+    ; di = src tte
+    call get_tte_typed_metadata
+    ; al = register typed metadata
+
+    and al, 11b ; al = register width
+
+    cmp al, 00b          ; 8 bit
+    je .unexpected_token ; TODO handle 8 bit opcodes
+
+    cmp al, 01b ; 16 bit
+    je .operator_2_memory_register_16
+
+    cmp al, 10b                             ; 32 bit
+    je .operator_2_memory_register_continue ; default register length; no prefix
+
+    cmp al, 11b ; 64 bit
+    je .operator_2_memory_register_64
+
+  .operator_2_memory_register_16:
+    mov al, 0x66
+    call .push_byte
+    jmp .operator_2_memory_register_continue
+  .operator_2_memory_register_64:
+    mov al, 0x48
+    call .push_byte
+    jmp .operator_2_memory_register_continue
+  .operator_2_memory_register_continue:
     ; si = r/m; dst tte
     ; di = reg; src tte
     mov dl, 00b ; dl = mod bits
@@ -283,6 +352,32 @@ assemble:
     call .write_byte
     pop rdi ; di = dst tte
 
+    ; di = token table entry
+    call get_tte_typed_metadata
+    ; al = register typed metadata
+
+    and al, 11b ; al = register width
+
+    cmp al, 00b          ; 8 bit
+    je .unexpected_token ; TODO handle 8 bit opcodes
+
+    cmp al, 01b ; 16 bit
+    je .operator_2_register_16
+
+    cmp al, 10b                      ; 32 bit
+    je .operator_2_register_continue ; default register length; no prefix
+
+    cmp al, 11b ; 64 bit
+    je .operator_2_register_64
+  .operator_2_register_16:
+    mov al, 0x66
+    call .push_byte
+    jmp .operator_2_register_continue
+  .operator_2_register_64:
+    mov al, 0x48
+    call .push_byte
+    jmp .operator_2_register_continue
+  .operator_2_register_continue:
     mov si, di ; si = dst tte
 
     call .next_token
@@ -316,9 +411,27 @@ assemble:
     call .next_token
     jge .break
     call .get_next_tte
+    ; di = tte
+    call get_tte_typed_metadata
+    ; al = register typed metadata
 
-    ; si = r/m; dst tte
-    ; di = reg; src tte
+    and al, 11b ; al = register width
+
+    cmp al, 10b ; 32 bit
+    je .operator_2_register_memory_32
+
+    cmp al, 11b                             ; 64 bit
+    je .operator_2_register_memory_continue ; default addr length; no prefix
+
+    ; other cases: 16 bit, 8 bit both are not valid for addressing
+    jmp .size_mismatch
+  .operator_2_register_memory_32:
+    mov al, 0x67
+    call .push_byte
+    jmp .operator_2_register_memory_continue
+  .operator_2_register_memory_continue:
+    ; si = reg; dst tte
+    ; di = r/m; src tte
     push rsi
     mov esi, edi ; si = reg; src tte
     pop rdi      ; di = r/m; dst tte
@@ -330,8 +443,106 @@ assemble:
     jmp .loop_next_token
 
   .operator_2_register_register:
+    push rsi
     mov rsi, .msg_operator_2_register_register
     call print.debug
+    pop rsi ; si = dst tte
+
+    ; di = tte
+    call get_tte_typed_metadata
+    ; al = register typed metadata
+    and al, 11b ; al = register width
+
+    cmp al, 00b          ; 8 bit
+    je .unexpected_token ; TODO handle 8 bit opcodes
+
+    cmp al, 01b ; 16 bit
+    je .operator_2_register_register_16
+
+    cmp al, 10b ; 32 bit
+    je .operator_2_register_register_32
+
+    cmp al, 11b ; 64 bit
+    je .operator_2_register_register_64
+  .operator_2_register_register_16:
+    ; 16 bit
+    push rdi
+    push rsi
+    mov edi, .buffer_end - .buffer ; length of buffer
+    mov rsi, .buffer               ; buffer location
+    mov dl, 0x66
+    call elemb
+    pop rsi ; si = src tte
+    pop rdi ; di = dst tte
+    cmp al, 1
+    jne .size_mismatch
+
+    ; 64 bit
+    push rdi
+    push rsi
+    mov edi, .buffer_end - .buffer ; length of buffer
+    mov rsi, .buffer               ; buffer location
+    mov dl, 0x48
+    call elemb
+    pop rsi ; si = src tte
+    pop rdi ; di = dst tte
+    cmp al, 0
+    jne .size_mismatch
+
+    jmp .operator_2_register_register_continue
+  .operator_2_register_register_32:
+    ; 16 bit
+    push rdi
+    push rsi
+    mov edi, .buffer_end - .buffer ; length of buffer
+    mov rsi, .buffer               ; buffer location
+    mov dl, 0x66
+    call elemb
+    pop rsi ; si = src tte
+    pop rdi ; di = dst tte
+    cmp al, 0
+    jne .size_mismatch
+
+    ; 64 bit
+    push rdi
+    push rsi
+    mov edi, .buffer_end - .buffer ; length of buffer
+    mov rsi, .buffer               ; buffer location
+    mov dl, 0x48
+    call elemb
+    pop rsi ; si = src tte
+    pop rdi ; di = dst tte
+    cmp al, 0
+    jne .size_mismatch
+
+    jmp .operator_2_register_register_continue
+  .operator_2_register_register_64:
+    ; 16 bit
+    push rdi
+    push rsi
+    mov edi, .buffer_end - .buffer ; length of buffer
+    mov rsi, .buffer               ; buffer location
+    mov dl, 0x66
+    call elemb
+    pop rsi ; si = src tte
+    pop rdi ; di = dst tte
+    cmp al, 0
+    jne .size_mismatch
+
+    ; 64 bit
+    push rdi
+    push rsi
+    mov edi, .buffer_end - .buffer ; length of buffer
+    mov rsi, .buffer               ; buffer location
+    mov dl, 0x48
+    call elemb
+    pop rsi ; si = src tte
+    pop rdi ; di = dst tte
+    cmp al, 1
+    jne .size_mismatch
+
+    jmp .operator_2_register_register_continue
+  .operator_2_register_register_continue:
 
     push rsi
     mov esi, edi ; si = reg; src tte
@@ -362,6 +573,11 @@ assemble:
     call print.error
     jmp halt
 
+  .size_mismatch:
+    mov rsi, .msg_size_mismatch
+    call print.error
+    jmp halt
+
   ; procedures
 
   ; add the line `jge .break` after call site
@@ -385,10 +601,12 @@ assemble:
 
   ; al = byte to write
   .output_byte:
+    push rdx
     mov edx, [.next_output_byte] ; get output byte's address
     mov [edx], al                ; write byte to that address
     inc edx                      ; increment address
     mov [.next_output_byte], edx ; put output byte's address
+    pop rdx
     ret
   .next_output_byte dd OUTPUT_ADDR ; next empty byte in output
                                    ; TODO get rid of this sketchy bit of state
@@ -401,38 +619,41 @@ assemble:
     push rax
     mov ecx, .buffer_end
     mov [.buffer_pointer], ecx
-    .push_byte_loop:
-      dec ecx
-      cmp ecx, .buffer
-      jl .push_byte_break
-      mov al, [ecx]
-      mov [ecx + 1], al
-      jmp .push_byte_loop
-    .push_byte_break:
-      pop rax ; al = byte to push
-      mov [.buffer], al
-      pop rcx ; ecx = old buffer pointer
-      inc ecx
-      mov [.buffer_pointer], ecx
-      pop rcx
-      ret
+  .push_byte_loop:
+    dec ecx
+    cmp ecx, .buffer
+    jl .push_byte_break
+    mov al, [ecx]
+    mov [ecx + 1], al
+    jmp .push_byte_loop
+  .push_byte_break:
+    pop rax                    ; al = byte to push
+    mov [.buffer], al          ; write desired byte to front of buffer
+    pop rcx                    ; ecx = old buffer pointer
+    inc ecx                    ; ecx = pointer to next empty in buffer
+    mov [.buffer_pointer], ecx ; record write .buffer_pointer
+    pop rcx
+    ret
 
   ; al = byte to write
   .write_byte:
+    push rdx
     mov edx, [.buffer_pointer]
     mov [edx], al
     inc edx
     mov [.buffer_pointer], edx
+    pop rdx
     ret
 
   .flush_write_buffer:
     push rcx
     push rax
-    mov ecx, [.buffer_pointer]
+    mov ecx, .buffer
+    dec ecx
     .flush_write_buffer_loop:
-      dec ecx
-      cmp ecx, .buffer
-      jl .flush_write_buffer_break
+      inc ecx
+      cmp ecx, [.buffer_pointer]
+      jge .flush_write_buffer_break
       mov al, [ecx]
       call .output_byte
       mov byte [ecx], 0x00
@@ -448,6 +669,7 @@ assemble:
 
   .msg_unexpected_token db "unexpected token, aborting", 0x0A, 0x00
   .msg_unsupported_memory_access db "unsupported memory access, aborting", 0x0A, 0x00
+  .msg_size_mismatch db "size mismatch, aborting", 0x0A, 0x00
   .msg_operator_0 db "operator_0", 0x0A, 0x00
   .msg_operator_1 db "operator_1", 0x0A, 0x00
   .msg_operator_1_memory db "operator_1_memory", 0x0A, 0x00
@@ -580,21 +802,21 @@ get_ModRM:
   .continue:
     shl ebx, 3
 
-  mov edi, esi ; do the other one
+    mov edi, esi ; do the other one
 
-  ; di = tte
-  call get_reg_bits
-  ; al = reg bits
+    ; di = tte
+    call get_reg_bits
+    ; al = reg bits
 
-  mov ecx, eax
+    mov ecx, eax ; cl = r/m bits
 
-  xor eax, eax
-  or eax, edx   ; mod bits
-  or eax, ebx   ; reg bits
-  or eax, ecx   ; R/M bits
-  and eax, 0xFF ; mask for byte
-  pop rbx
-  ret
+    xor eax, eax
+    or eax, edx   ; mod bits
+    or eax, ebx   ; reg bits
+    or eax, ecx   ; R/M bits
+    and eax, 0xFF ; mask for byte
+    pop rbx
+    ret
 
 ; ------------------------------------------------------------------------------
 ; get_opcode
@@ -1307,7 +1529,6 @@ identify_register:
 
 ; ------------------------------------------------------------------------------
 ; identify_operator
-; TODO combine with identify_register
 ;
 ; description:
 ; takes an operator in ascii-encoded text and returns its token ID or
@@ -1660,6 +1881,86 @@ tokens:
     db 00010110b ; reg: 101b
                  ; width: 10b (32 bits)
 
+    dw 0x0020    ; ax
+    db 0x02      ; type: register
+    db 00000001b ; reg: 000b
+                 ; width: 01b (16 bits)
+
+    dw 0x0021    ; bx
+    db 0x02      ; type: register
+    db 00001101b ; reg: 011b
+                 ; width: 01b (16 bits)
+
+    dw 0x0022    ; cx
+    db 0x02      ; type: register
+    db 00000101b ; reg: 001b
+                 ; width: 01b (16 bits)
+
+    dw 0x0023    ; dx
+    db 0x02      ; type: register
+    db 00001001b ; reg: 010b
+                 ; width: 01b (16 bits)
+
+    dw 0x0024    ; si
+    db 0x02      ; type: register
+    db 00011001b ; reg: 110b
+                 ; width: 01b (16 bits)
+
+    dw 0x0025    ; di
+    db 0x02      ; type: register
+    db 00011101b ; reg: 111b
+                 ; width: 01b (16 bits)
+
+    dw 0x0026    ; sp
+    db 0x02      ; type: register
+    db 00010001b ; reg: 100b
+                 ; width: 01b (16 bits)
+
+    dw 0x0027    ; bp
+    db 0x02      ; type: register
+    db 00010101b ; reg: 101b
+                 ; width: 01b (16 bits)
+
+    dw 0x0030    ; al
+    db 0x02      ; type: register
+    db 00000000b ; reg: 000b
+                 ; width: 00b (8 bits)
+
+    dw 0x0031    ; bl
+    db 0x02      ; type: register
+    db 00001100b ; reg: 011b
+                 ; width: 00b (8 bits)
+
+    dw 0x0032    ; cl
+    db 0x02      ; type: register
+    db 00000100b ; reg: 001b
+                 ; width: 00b (8 bits)
+
+    dw 0x0033    ; dl
+    db 0x02      ; type: register
+    db 00001000b ; reg: 010b
+                 ; width: 00b (8 bits)
+
+    dw 0x0034    ; sil
+    db 0x02      ; type: register
+    db 00011000b ; reg: 110b
+                 ; width: 00b (8 bits)
+
+    dw 0x0035    ; dil
+    db 0x02      ; type: register
+    db 00011100b ; reg: 111b
+                 ; width: 00b (8 bits)
+
+    dw 0x0036    ; spl
+    db 0x02      ; type: register
+    db 00010000b ; reg: 100b
+                 ; width: 00b (8 bits)
+
+    dw 0x0037    ; bpl
+    db 0x02      ; type: register
+    db 00010100b ; reg: 101b
+                 ; width: 00b (8 bits)
+
     dw 0x004F ; hlt
     db 0x01   ; type: operator
     db 0x00   ; # operands
@@ -2041,6 +2342,10 @@ whitespace_2 db " ", 0x0D
 ; test program
 program:
   db "xor eax, eax", 0x0A
+  db "mov rax, rax", 0x0A
+  db "mov rax, rbx", 0x0A
+  db "mov eax, ebx", 0x0A
+  db "mov ax, bx", 0x0A
   db "inc rax ; inline comment", 0x0A
   db "dec rax", 0x0A
   db "; one line comment", 0x0A
@@ -2048,6 +2353,9 @@ program:
   db "mov [rax], rdx", 0x0A
   db "mov [rcx], rbx", 0x0A
   db "mov rcx, [rbx]", 0x0A
+  db "mov rcx, [ebx]", 0x0A
+  db "mov ecx, [ebx]", 0x0A
+  db "mov cx, [ebx]", 0x0A
   db "hlt", 0x0A
   .size dq $ - program